Upgrading your cards will significantly increase their power! Your existing cards or acquire new ones by opening packs that are purchased using coins or clams. Both coins and clams are earned as rewards in regular events, adventure stages and challenges, for completing quests for donation, or through card recycling.
There are other ways to get new cards, but those have higher chances of getting rare/epic/legendary rares than non-card purchases. Questing is the best way to play many cards at once. Save your Quest Tokens to get 10, 20, or 50 at a time.