A you level up and win extra cards, choose your best cards locked so you do not use them by mistake and only level them up. Leveling a card costs money that should not be wasted that on a card that you will stop using soon. Usually, the more stars a card has, the better it is for your squad (especially because it has a higher maximum level) As soon as reach maximum level with a card, you can evolve it if you have the required cards. Evolve your cards as soon as possible to give them more power and room to grow.