To make progress quickly, start off completing the quests to maximize your uranium, titanium and credits, and to earn free crystals very quickly.
When your resources are maxed out start building and upgrading your warehouses and your banks. Spend it on researching new technologies and on training new troops in the barracks.
When searching for bases use the recon feature to figure out which bases have been abandoned.
Always use abandoned bases to farm it and use it for resources by attacking it over and over, without the worry of being beaten.
If you want to attack active bases, stick to the ones that are a similar level to your base.
Build the air field, the military factory, the S.C.A.R. training complex and the battalion headquarters as soon as possible in order to train more powerful troops.