Playing through the Campaign missions will be your first opportunity to earn resources to build structures to expand your base, and build units to grow your army. Campaign missions will give you some much-needed practice with deploying and controlling your units in battle. Go through as many missions as possible early to rack up tons of metal and oil. However, be sure to build warehouses and oil storage back in your base to make sure you have room to bank all your resources. In fact, you may want to wait to complete a mission until you run out of Oil.