Creating decks in Hearthstone is fairly straightforward. But you can't just walk in and put any pile of cards together and expect it to be able to compete. First you select the class you want to play. You are then restricted to only use the cards that belong to that class along with cards available to all classes. Just about all of the cards in Hearthstone can be divided into three major categories: Removal, utility, or minions.
Most well rounded decks will feature elements of all three.