I think perhaps the best way, though I too am just starting out, is to give a quote based on an estimate of the hours you expect the project to take you, at your hourly rate, plus some additional padding in case of scope creep. For instance, if you charge $40 an hour, and expect the project to take 10 hours, you may charge $500 or so.
SitePoint is giving away a free ebook on the Principles of Freelancing; I’ve been reading it studiously for the past few days as I have time. It is an extremely informative and helpful source of information for us just-getting-started freelancers. The Principles of Successful Freelancing
Hope you don’t mind my posting the links, mods!