Usually you do what you want to earn per year divided by amount of billable hours per year, and add a bit for expenses and overheads.
but really if your just starting out you need to get as much work as possible to make a name for yourself, whilst not underselling yourself - and doing an extra good job to get referals.
take into account where you are, if you already have clients or are starting from scratch, your experience etc.
(if your in the city you'll be able to charge alot more than if your in a small town to start with.)