I am writing a letter to my manage about reducing my hours so I may go to college what should I include?

Make it brief. Be clear about what hours you will work and what they will be. Make them as regular as possible. A boss doesn't want to have to wonder every quarter when you will be available. If the college work will help you as an employee, mention that as well. So effectively:

Dear boss, I need to reduce my hours because I will be taking classes at X college. I will be available to work X hours per week (then list what hours you will be available). My degree will be in X, which will help me be a better, more knowledgeable employee for X company.

If your hours will vary term to term, let the boss know you will keep him advised as soon as possible what the hours will be each term. Or reassure the boss that your hours won't change term to term.

I absolutely agree. Be straightforward and honest with your employer. Show that you can be dependable. And when you give your availability, make sure that you can commit to it. Your boss will be very irritated if you say you can work at 4:00 but then you find out with rush hour traffic you can't get there until 4:30. Try out your proposed schedule before requesting the change to see if it works for you.