One thing I wish I did for myself - if I could go back in time and start over as a freshman in high-school - is look up essay prompts for college applications and begin thinking and writing about these prompts. Starting early will give you more time to think, reflect, and revise your essays. Of course, the prompts are different every year, but it's easy to accommodate college essays to many different prompts. Here is one of them from CommonApp this year:
Think about an incident or time when you experienced failure. How did it affect you, and what lessons did you learn?
When you have a lot of time and no pressure to consider this prompt, you will be able to expand beyond what a normal college applicant would say. You may realize how bad you are at writing, but that's why you are doing this before it becomes a necessity - to be prepared.