First thing, if you want to play you're gonna have to go to spring practice. Every football team I know of holds spring practice for about a month sometime between March and May. This means you need to plan on only having about 6-7 months to train.
That being said, at 5'9 160, you probably won't be able to play anything besides a skill position. This means you need to get FAST. Speed is the name of the game here. If you can catch, you have more options because you could play receiver or running back. If you can't catch a cold then you're limited more to the defensive side of the ball at either cornerback or safety.
Either way, you need to be getting your legs strong and running sprints. By getting your legs strong, I don't mean your quads. I mean your hamstrings and glutes. You want to focus on posterior chain exercises.
Try to gain as much muscle as possible, but realistically it'll be tough for you to show up in spring weighing more than 175 without having gained too much fat.
If you need help organizing your training, let me know. I've been training to walk on to my college team now for about 6 months and it's been a learning process the entire time. Try playing alot of basketball in the meantime as that will help with your agility, reaction abilities, and hand-eye coordination.