The short answer - by sprinting!
Think about all of those drills you do in football. Notice they have something in common - explosiveness. They require you to go from rest to full effort, as fast as you can. Try to recreate some of those drills for your off season training, especially things like jumping onto boxes, agility drills, short sprints, etc.
Your 40 doesn't tell people a lot, since there's no standard. Different coaches measure them differently - on the track, on grass, in cleats, in spikes, in flats. The 100m, however, is measured on the track, and is directly comparable. A 11.6 is nothing to sneeze at for a freshmen. Keep working hard at track practice, keep working hard at football practice, and you'll keep improving.