Getting cards is a random process. Explore the single player areas to encounter other players to earn friend points. You can spend 200 friend points to draw a random one to four star card. Merge your cards to improve them. Select a base card to improve, then sacrifice unneeded cards. You will get duplicates of cards you already own as you explore, and they will automatically level up. Combining several cards in the same attack row to activate a combo. Combos provide passive stat boosts to all your cards.