• Forget everything you've ever learned about writing. Well, maybe not everything - but forget the formal tone you've had to perfect over your years as a student. Commercial writing is typically conversational, casual, and (yikes!) not always grammatically perfect. Use contractions, first and second person, and words nobody needs a dictionary to understand.
• Get onto a job board and find some projects. This is the quickest way to get your first paying gig. It can take some persistence, though - don't just place a few bids and bail. You've got to stick with it for a while before you get that first project. I recommend Guru, since I'm most familiar with it. Other boards are RentaCoder, Elance, and a few others out there.
• Take it seriously. Your client isn't going to care that studying for your exam made you late in submitting your project. If you're a freelance writer, you're a professional. Act like one. Nobody likes excuses for lateness.
• Build a portfolio. Keep all your writing samples. They'll come in handy later! After you've done a job, make sure you save it as a PDF to use in your portfolio.
• Keep it ethical. You will absolutely get caught if you plagiarize, so don't even consider it. (This goes for your course work, too!) Just because you're not "out" in the workforce, doesn't mean what you do doesn't count. You should run your business with all the integrity you'd want to demonstrate in your future career.

Freelance writing is a great way to offset those hefty college expenses. And you'll never have to ask, "Do you want fries with that?"