Tap on your character card, then select SKILL TREE. The tree is split into 5 sections: (Clockwise from the left) Attack, Character Abilities, Energy, Defense, Equip Slots.
The attack section is on the left. In there you can raise your base attack value, your crit rate value and your crit damage value.
U can also drop Moves (Blockbusters or Special Moves) that provide Ur wanted stat and equip them. The stat from Move can be increased by upgrading (increasing level) that Move.