Your letter needs to be very professional, free of errors and clear about your interest. You need to make the team believe that they need you by what you write.
Professional football teams hold an open draft where you can show off your skills. If you have outstanding stats that you feel have gone unnoticed, you can detail them in a letter, but showing up to prove your mettle is key.
You start out your letter as if you were writing to anyone in a letter. Use to whom it may concern, or to whichever football team your interested in. Write the body, and then end it with a sincerely and your name.
If you're looking to try out for them, they probably list their try out dates on their website. It's extremely rare to have them contact you from a letter alone.
