I would take on a few projects on sites like Scriptlance first and build up some reputation first before quitting your regular job.
When you start out you will probably have to bid lower to get projects, but as you get more feedback and reputation you can start going after larger projects and charge more.
Eventually of course this may lead to you having enough work to leave your regular job. It's much better to do it this way rather than quitting all at once and then having to worry about paying all the bills at the same time as trying to build up your freelance career.
Hope this helps
