Price depends on: size of customers, experience, quality, what competition charges, demand, etc.
Don't expect to be billing 40 hours a week, especially at the beginning.
Be sure to add 5% for vacation time.
Be sure to charge tax if you have to pay it.
If you worked 40 hours a week, and wanted to make $50,000 per year, you'd charge $25/hour. But you likely wont have 40 billable hours per week, so to make that salary you may want to charge $30-$40 per hour.
Use time tracking software like ziptimetracker.com to keep track of your billable hours and send invoices to your clients at the end of the month.