The first thing to note when building a deck is the number of cards, no more and no less than 60 cards are required for your deck.
The second rule to note is that decks that are played require no more than 4 of each card with the same name, this means if you have 4 Charizard’s from Base Set, you can’t have any Charizard from any other set, unless they have a different name. Cards with levels in the name, for example Luxray Lv X, are the same card as Luxray, despite the level.
This rule however does not work for cards with symbols, these symbols are part of the cards name and so you can have 4 cards with Alakazam ☆, 4 cards Alakazam and 4 cards Sabrina's Alakazam.
The final rule to consider is you MUST have at least one Basic Pokemon in your deck.
An important new rule, with the introduction of Radiant Pokemon in Astral Radiance, only one Radiant Pokemon can be used per deck, however as they are basic pokemon they can be used straight from a hand and do not require any previous Pokemon to evolve from.