There are 2 options to get more credits.
1. Buy credit packs. Simply tap the coin bar on the top right of the app:
2. You can also earn free credits by downloading apps. Start by tapping the coin bar on the top right of the app:
Then, tap on "Earn Free Credits"
You will be presented with offers. Look for the simpler ones like "Install and RUN". These offers are easier to complete and you get the credits faster.