Collecting Pokemon cards can be as easy, as hard, as cheap, or as expensive as you want to make it. I personally go for one of every English card released, but some people only go for certain sets, artists, Pokemon, types of cards, championship or promo cards, a card for each Pokemon, and so on. It's just what appeals to you. There's lots of ways to collect. 

So where do I start? Well, it depends on what your goals are. I've provided some tips for various audiences who have an interest in playing or collecting Pokemon cards:

General Tips for Everyone

• Buying packs is a losing proposition, money wise. It's fun, but expect to lose money opening packs for valuable cards and reselling duplicates. The odds are purposefully not in your favor, and rare does not always mean valuable in the Pokemon TCG world, especially for newer sets.
• Buying lots from eBay will get you more cards for cheaper, but it's not advisable if you're looking for mint condition cards or several specific cards. YMMV.
• Buying cards you need from TCGPlayer or Troll and Toad is cheaper than buying packs of cards, but not necessarily cheaper than eBay lots or auctions.
• For newer cards that are still used in the Standard TCG format, usually card prices are not determined by rarity, but by how useful the card is in the TCG. Standard is by FAR the most popular TCG format, so skipping expensive cards now and picking them up a few months or years down the line when they are used less or fall out of Standard and into Expanded is a viable strategy. Exceptions to this rule include popular Pokemon, especially Charizard.
• Card stores can also be a great source for cards, but prices vary.

People Who Want to Acquire Lots of Cards for Cheap or Want Large Collections

• Typically, the cheapest cards are from 2015 and newer.
  • Sets released after Team Rocket (April, 2000) and before Primal Clash (February, 2015) are more uncommon than Standard Base Set (not 1st edition, shadowless, or additional), Jungle, Fossil, Team Rocket, and anything made in or after 2015.
  • For bulk cards, the most expensive cards are going to be Base Set 1st edition cards, Shadowless cards, and any cards printed when Pokemon was least popular (2002 - 2007).

People Who Want to Play the TCG

With one caveat, the maximum number of cards with the same name you're allowed to have in a deck is four (4). This applies to trainers and special energy cards. For monster cards, even if you have four different Pikachu cards from different sets, you can only have four cards with the name "Pikachu" in your deck. For example, you can have four different cards named "Gardevoir" in your deck and four "Gardevoir GX" cards in your deck.  You can have as many basic energy cards in your deck as you want. You must have 60 cards in your deck for it to be legal in tournament play. With those stipulations in mind:

• TCGPlayer and Troll and Toad are your friends. The cheapest way to build a deck by far is to buy singles. Research what you want to play, and then buy singles to make that deck.
• If you're patient, you may be able to get a better deal than TCGPlayer and Troll and Toad by looking on eBay for auctions of singles or of "playsets" containing four cards.
• Buy heavy duty sleeves with opaque backs for playing. I personally use Dragon Shield, but there are other good ones. A good set of sleeves will protect your cards and prevent you from having to replace cards. The Pokemon TCG (or any competitive TCG) frown upon marking cards, either accidentally or on purpose. As unprotected cards are played, they acquire wear and can become identifiable in a deck if they are not sleeved. This is, of course, considered cheating. Don't be a cheater.
• Consider buying a structured deck to learn the ropes of the game, or spend a little time playing Pokemon TCG Online, which is free. Both can help you get familiar with the game before playing against people at your local store or meetup.
• Buying a playmat can be helpful, but isn't required to play. 
• Consider buying one Elite Trainer Box (ETB). Today's ETBs come with:
  • Dice, which are used to count damage (1 = 10, 2 = 20, and so on).
  • A package of energy cards to use for decks.
  • Themed card sleeves, which are worse than Dragon Shield and other high-end card sleeves for playing, but are great for reducing damage to cards while they sit in boxes.
  • A GX or Tag Team GX token. A GX attack can only be used once a game. When it's used, the token is flipped over to indicate that a player's GX attack has been used. New cards from Sword and Shield (as of right now) do not have GX attacks, but a GX token (of some sort) will still be required until cards with GX attacks are no longer allowed to be played.
  • Burn and poison tokens.
  • A code for redeeming the same ETB in the online game, Pokemon TCG Online. The online game can be a great way to play fun decks or test decks for cheaper than buying cards.

People Who Want to Resell

Welcome to the world of graded cards! 

• With graded cards, you do have a better chance of breaking even or better if you get lucky and make good purchases. 
• People do buy both vintage and modern graded cards. Cards with higher grades will sell quicker and for more money. However, grading cards is popular, and the profit can be enough to support the hobby of buying more cards, but rarely much else.
• Unless a card is going to be a gem mint 10, cards worth less than $25-30 before grading are likely to lose you money.
• Most modern cards are not gem mint 10, even straight from the pack.
• Japanese cards are made better and tend to receive better grades. 
• Popular Pokemon, such as Charizard, are exceptions because their cards tend to be more valuable.

People Who Want to Collect the Rarest and Most Expensive Cards and Products

Welcome to the world of graded cards, Japanese cards, AND sealed product!

• Japanese cards are much, much more difficult and expensive to collect for anyone outside of Japan. A majority of the rarest and most expensive cards ever printed are Japanese, and they have always gotten lots of exclusive cards. Many Japanese-exclusive cards are very pricey (over $100) or very limited, and most are pricier than normal promos that also saw a release in North America.
• Japanese cards are made better and tend to receive better grades. 
• Other foreign language cards can be equally as difficult to collect for as Japanese cards, but are usually cheaper than English or Japanese cards.
• eBay is your friend, but also consider other avenues, such as other websites that may be in Japanese, sites that offer services that buy product in Japan and send it to you for a fee, or joining collecting forums. 
  • I'd recommend the Elite Fourum (http://efour.proboards.com/). The people on that board have lots of knowledge, means, experience, and clout in the hobby, especially for high-end Pokemon-card-related stuff. However, selling threads are not allowed there. 
• Be prepared to pay a lot for shipping. 

People Who Want to Collect Only Certain Cards

Some or all advice I have provided for other collectors can apply to you, depending on what you want to collect. I've provided some tips for various audiences who have an interest in collecting certain Pokemon cards:

People Who Want to Collect Cards for One Pokemon

It's Charizard, right? RIGHT? Everyone likes Charizard.

• TCGPlayer and Troll and Toad are your friends.
  • If condition matters, use eBay or other sites that provide pictures of the card you want to buy.
• If you're patient, consider looking on eBay for auctions for singles. You might be able to do better on pricier (Cost > $5 or so) individual cards if you wait for auctions.
• If you want to collect Japanese variants, or Japanese exclusives, or other language variants, the information in "People who want to collect the rarest and most expensive cards and products" applies to you. 
  • Also consider making your own spreadsheet. Full lists of every card of a Pokemon aren't easily accessible, and if you want to get into Topps cards, CARDDASS, Artbox, Japanese variants, Japanese exclusives, other language variants, card sleeves, and other types of Pokemon Cards, your best bet is gathering all that information yourself.

People Who Want to Collect Certain Sets

Gettin' those childhood sets back, eh?

• If you want a quick fix, check on eBay for complete sets. Buying a complete set can be cheaper than buying all the cards individually.
• If you want to find the best deals:
  • TCGPlayer and Troll and Toad are your friends.
  • If you're patient, consider looking on eBay for auctions for singles. You might be able to do better on pricier (Cost > $5 or so) individual cards if you wait for auctions.
• If you want to piece a set together:
  • eBay or other online marketplaces
  • Card stores for singles

People Who Want to Collect Certain Types of Cards, Like Promos or Championship Cards

"I like life on hard mode."

• If you're patient, consider looking on eBay for auctions for singles.
• Consider making your own spreadsheet. Full lists of every card type aren't easily accessible. There are lists out there, but depending on your scope, they might be incomplete for your needs. 
• Some of these cards can be expensive and rare, especially if you want to collect Japanese variants, or Japanese exclusives, or other language variants. The information in "People Who Want to Collect the Rarest and Most Expensive Cards and Products" may apply to you.

People Who Want a Card for Every Pokemon

*Pokemon theme song starts playing*

Are you picky? If you are, use TCGPlayer, Troll and Toad, and eBay to get the cards you want. eBay especially if you condition matters. If you're not picky, a buying a couple card lots from eBay can put you well on your way to that complete Pokedex.

If condition matters, use eBay or other sites that provide pictures of the card you want to buy.

People Who Want to Collect Cards by a Certain Artist

The information in "People Who Want to Collect Cards for One Pokemon" applies to you. Get your inner Bob Ross on and fill a binder with your happy Pokemon.

People Who Want to Collect Cards That Look Like They've Been Through a War

"I've seen some things, man."

eBay lots. All day. Even when buying lots with good condition cards, you're bound to get a few. 

People Who Want to Collect Fake Cards

China has entered the chat

The best way to find fake cards is eBay (because some fakes are moderately valuable and have unique holo patterns), card lots (because lots usually contain a few fakes), cards shipping from China, and local places where lots of fake or knock-off items are sold. If an eBay lot looks too good to be true, it might be for you.