To expand on ElYusubov's suggestions, remember to always include a 'buffer,' especially when charging a flat rate.
I've only been doing web development freelancing for a few months, but I learned quickly that what seems like a 2 hour job quickly escalates to a 10 hour job. Clients will forget to mention things, features you thought would suffice can end up not working, etc.. Get as many details out of your client as you can early on; you'll be less likely to get sucked in an endless project, and your clients will likely notice your professionalism.
Last, freelancing isn't exactly a gravy-train. While you can get experience beyond your years very quickly, you can't expect to make much. Instead, you gain popularity, reputation, wisdom, and very nice bullet points on your resume. You might still want to look for a solid job in addition to freelancing.