There are two official ways to play the Pokémon TCG: with the actual cards and via the Pokémon TCG Online. When you get firsthand product (like buying an unopened booster pack), you actually get
both because there should be a card with a redemption code for the online game in it.
To begin your real-world card collection, I recommend finding a recent Battle Arena Deck. This is a series of paired, preconstructed decks that typically have an MSRP of $29.99 USD. The decks within are reasonably balanced against
each other, but I'm not sure if they are against much else. The Pokémon in these are usually only legal for
Expanded play, but you'll get some important Trainer cards that work for either
Expanded or
Standard play. If you are wondering what on earth those two things are, then I'll add this page might be a good place to do some reading; it is the
official website's Rules & Resources section, and you can start with an instruction booklet. Asking questions here is good as well.
An area I can't help you out with is... well... the next step. You'll need to learn how to spot fake cards, scams, and even for the legitimate stuff learn how to find good prices and trades. Trading well is
vital if you don't want to the game to cost you an arm and a leg. XD
The PTCGO is a bit different; while you can redeem those codes from products, and
in some areas spend money directly on an in-game currency that can then buy digital product, there is a
third option and it is one I've been using for a few years. The PTCGO has a
lot of in game rewards, and while you'll still
need to learn how to trade I find it a bit easier there. With the PTCGO, you just need to take the Theme Decks they provide you, take them through the Trainer Challenge (single player) mode to ensure everything within them is unlocked, plus earn some easy, early rewards, then select your best Theme Deck and start grinding for rewards in the Theme Deck Format of the VS Mode. Eventually, you enter Tournaments (found under the Events tab) once you have enough for the entry fee, and start earning booster packs you can trade with other players (as opposed to "locked" ones you can only hold onto or open). Once you've got those, you trade for the cards you need. The current Reward Ladder looks pretty good right now, actually, as it contains
Puzzle of Time and
Volcanion-EX. I apologize, though, if this mode doesn't interest you; however, that means your own redemption code cards can be set aside to sell or trade.
