What do I do to get credits? They eventually go into negative and then I can't build anything
It seems that 1 raw oil node, 1 ore node, 1 metals node and 1 plastics node are anough for money to go up if you switch everything to sell, no matter how many buildings I have, I don't remember what I did the first time. Note though that inexperienced player can softlock themselves if their maintance cost is higher than everything they can sell. Can I ask what things do have a maintance cost?
Fighter planes should just be in the airstrip, assuming they're in your loadout. The mobile SAM system is a land vehicle. The SAM turret is an upgrade in the base turret. You might be playing an older version, the loadout system was updated somewhat recently so there's a lot more structure now to the unit types you start with.