The game has changed a lot. Reading up on the new rule book will help you with the basics of what cards you can play on the first turn. Level X cards have been introduced, and can be played onto existing cards of the same name. Supporters have been introduced, which are cards sort of like Professor Oak or The Boss's Way, and you can only play one supporter a turn.
Stadiums are now in effect as well, basically giving us 3 distinct types of Trainers. Trainers, Supporters, and Stadiums.
Stadiums, once played, stay in play until another one is put down, and affect both players at once.
This is just the start of the changes that have been introduced, but I'd definitely recommend buying a starter deck and reading up on the new rules.