Pokemon rotates regularly, and because of this at tournaments you will see two formats:
Standard (the newest sets)
Expanded (the newest sets and some older sets)
Format details can be found here:
Coming in as a new player you are given three theme decks. These Decks use a very basic format (20 pokemon, 20 trainers, 20 energy). This is not a good format to play with and not a good basis to learn deck sizing on.
However, there is a set of trainer challenges infront of you when you start, beating all of the trainers in each level 3 times (its a bit of a grind) will set you up with enough gold and tradelocked packs to ensure you are in a good position card wise. On this note I must state it is very easy to F2P on this game compared to other online TCG/CCG’s.
Once you have done this there is a lot of ways to earn in game currency and packs. Every 20 days there is a ladder, with three tiers of points going up to 2000.
Tier 1 is usually small rewards with the end being an EX pokemon (sometimes staple, sometimes not). Tier 2 is a more stacked version usually with a line of evolutions in the middle and the final tier will have 8 packs, 10 tournament tokens, 600 coins, a gold chest and a full art trainer (usually high value, not always staple).
There is also a daily ladder that rewards you with coins and token and when special events are on you can sometimes even get like 700 coins from one win.
I must add that there is streaking in the game, 10 points for 1 win capping at 30 after 3, plus 1 coin for each prize you take.
One season I finished the 1st tier of the reward ladder in 10 games despite it being 210 points!
Decks can be very simple and there are some great free to play decks that dont need any expensive cards to be viable. Pre sun and moon an example of this was Regirock, who was immune to EX pokemon, so I would use him to grind the ladder and hard counter any EX’s while I built up. The same can be said now for hoopa/alolan ninetales who are immune to GX.
There is a tonne of resource online for deck building, as well as me and @Rod internally who can help with everything.