From Starfleet Commander
Credits are the form of currency in Starfleet Commander. They enable you to buy things from the Store Page such as Commanders, Merchants, or Basic Androids, and can be used to complete research or buildings instantly.
You can get them by clicking "Credits" in the upper right-hand corner, or by clicking the button in the first box on the Store Page.
On the Credits page, you can either purchase them directly with a credit card or PayPal account, or you can earn them by doing offers.