Tap the cards that you need in order to break the seal and select their source. You can see where they are obtained. Most of the cards cannot be obtained immediately. You will need shards in order to complete them. You can determine which level has their shards. When you first complete a level, you will be given five free "peeks". This allows you to get the rewards of the level without actually having to fight a battle.
Cards
Keep the strongest cards in your deck at all times. The number at the bottom right corner of each card generally indicates its strength. The higher the number, the better. Have at least one healer card in your deck. At the beginning the number of cards you can bring into battle is limited. As you level up, you will unlock more slots. Make sure to always bring a healer card. Your attackers will go down quickly if one is not available. Break seals to free powerful creature cards after completing level 2-5. Removing seals gives you an powerful card, but you will have to sacrifice five cards in the process. The first card you can free is Thalassa, the dragon. Power up your important cards by fusing essence cards or other extra unwanted creature cards into tehm. Depending on the materials used, the base cards will receive experience and level up to become stronger.