Don’t neglect Mother Base.
Once you’ve played through a few missions, you’ll have access to Snake’s HQ, Mother Base, which you can manage through a series of convoluted menus. It’s kind of tempting to ignore these menus, but that’s a bad idea—as you play, be sure to keep managing your staff, expanding your base platforms, and developing new weapons. Once you unlock combat units, be sure to keep them deployed as often as possible, too—the more you participate in the game’s economy, the better off you’ll be.