There are 13 missions in chapter 1. The first few missions should be pretty easy to complete. If you are having difficulties with completing some of the later missions, replay some of the first few missions in "mission", to earn some extra credits. (completing these on higher difficulty gives you more credits). Then use these credits to order things from the HQ, making it easier to complete a mission you find difficult to complete.
-Keep moving while in combat to avoid bullets.
-Use grenades/rockets/airstrikes to take out towers, bunkers, tanks, etc.
- pick up the health packs only when you need them. If you don't need them, return at a later stage when you do need them.