Best place to start:
Identify your annual outgoings and work out what you would be happy to earn per annum after you have removed these costs (remember to factor in tax).
Work out what this equates to per hour, what ever you charge scale it out to a full-time wage (early on you won't be able to get enough work to do eight hours a day though) and ask yourself are you satisfied with that?
The tough bit is working out what you can charge for your experience, get some quotes from local freelancers and see how you compare. Consider pricing yourself a little higher to give the impression that you are better, you don't want to compete on just price.
Finally, never accept a job because something is better than nothing. You have to be motivated by the reward and if a company is offering a low reward, work quality will decrease and you'll find it difficult to increase your rates with that company.