For starters, in the initial deck builder, press Down until you come to a White card and press Fire. Do this again to fill your deck. Then press Right to start the game.
Your job is to play cards on the table and use them the next turn to attack your opponent. If the opponent has cards on the table as well you need to take those cards out first.
Each card has a cost (upper left), attack value (bottom left) and defense value (bottom right). An attack subtracts the attack value of the attacker from the defense value of the defender.
The first turn you start with 1 point to spend on cards, and that increases each turn.
The cards that you can use on a turn are highlighted. Click End to end your turn.