Credits are the main currency in the game.
The easiest way to earn credits is by upgrading production and supply buildings in your base. That way you ensure a regular income.
Successful attacks on the GOR-Alliance will always earn you a good amount of credits.
Mine resources in the darker places on the map, ensure you a nice but risky additional income.
To make a quick buck, attack other players in PvP mode via the button in the lower right on the menu and win the fight.