In addition to what others above have recommended, I would start a writing group. It took me a while to get a solid group of people together, but once our group was formed, I found it incredibly rewarding. You'll learn how to apply other people's critiques, apply feedback, and essentially develop a thick skin, which is needed as a writer. Once trust is built, you'll know you when praise is genuine and also where your weaknesses are. But you will also provide a great deal of feedback, and assessing other writers' work will improve your own skills. It will also force you to meet deadlines as you'll be required to get work in to be reviewed on time. Lastly, the group will become the beginning of a network to learn about other opportunities as you all find successes in different areas.
As for college, I think you should focus on what you want to write about in addition to the mechanics of writing itself. I have a degree in history which has provided good background for fiction writing (although I don't recommend getting a degree in history for that alone, as it's expensive and there are cheaper ways to learn about ancient Rome or medieval England).
I also recommend starting small--if you're into fiction, don't jump write to an epic novel. Start with short stories and see what sort of success you have there.
Thomas recommends the following next steps:
- Start a writing group.
- Work on specific goals, like participating in writing contests or submitting to specific publications