So. What i usually do and always tell people to go about it is the following:
There are two ways to go about it:
1. Just go into the internet and search for decks that work and buy the cards you need for it
2. Find a card you think would be really fun to play. Whatever that might be. And look for cards that would compliment it.
So you have the "Main Pokémon" (probably 3 or 4 times) then you have Energy (depending on the deck about 7-15) and some Trainer Cards you should have in every deck anyway, Draw Supporter (Prof. Sycamore, Shauna, N, Lilly what evs.), Stadium and a few depending on how much money you wanna spend. (You want Trainer's Mail, Max Elixir, VS Seeker etc in your decks usually but they are a bit pricey)