First you need a good number of energy cards your deck. Since you are only allowed to have 60 cards total in your deck, you need to pick each card strategically. Anywhere from 25-35 of the cards in your deck should be energy cards.
To win at the pokemon trading card game you need to have a lot of basic pokemon. An average deck should have about 10-15 basic pokemon depending on your strategy. If all your pokemon are second and third level, you may never get a chance to play them!
That being said, you want to have 2 or three really strong pokemon in your deck. These will usually be the second or third evolution, but make sure you have enough basic pokemon to play them! These will be the powerhouse of your deck. Every deck needs a few of these to be able to compete. The goal is once you get this out, you basically win the game.
Work with the same pokemon. Having 2 or 3 of the same card in your deck is not a bad idea—in fact it just may help you win your pokemon card match.
If you are have a pokemon in your deck that needs to evolve twice, you should have 4 of the first evolution, and 2-3 of the second. You might have another 1-3 of the final evolution depending on the strategy of your deck.
Finally, use trainer cards wisely. Don’t underestimate their importance, they could just give you the edge in the match and help you win your pokemon battle!