Read up on the college. You're going to have to convince people that you love this college and that this college would love you. If you were trying to convince someone to date you, you'd learn a lot about them in order to be able to talk about what you have in common. Applying to college works the same way.
Write about why you want to attend the college. Be careful not to sound like a ridiculous brown-noser (e.g., "I think Ohio State is the best college in the whole world and I've wanted to go there my whole life!"). Try to honestly talk about the things that attracted you to the college. After all, if you didn't really want to go there, you wouldn't be applying.
Talk about why you're just the type of student that the college is looking for. Obviously, you should mention your straight "A" average or any other academic qualifications. But this isn't just about academics. If the college has a well-known theater department or sports team, and you excel in those areas, mention that also. If the college brochure talks about wanting people who work with the community to make the world a better place, and you spend your weekends volunteering in the community, be sure to talk about that.
Explain that you have something unique to offer. Once you're done talking about how you possess what the college is looking for, point out a special feature of yours that might be what the college needs. This can be anything from a talent on a musical instrument to a burning desire to found a humor magazine at the college. Thousands of students with good grades will be applying, so your job here is to show why you're different and why you'll be an asset to the college.
Spell-check. Then spell-check again. Then have your parents read over the letter. Then check the letter again yourself. This may seem excessive, but getting into college is difficult enough without you hurting your chances even more with a badly spelled letter.