you are just starting out, so it's ok to charge less than the big firms until you feel that your work and experience are up to par - which may not take very long. but make sure that you are being paid a fair price for the work you are doing - do not be taken advantage of - you are still performaing a valuable service. I would charge monthly to maintain the website - if you are doing work on it regularly - otherwise you will get to a point where you're doing work for nothing and will start resenting it.
as to how much, that's up to you. ultimately, you have to feel comfortable with the amount you charge, whether it's a high or low number.