It will be my first job and it will be minimum wage,

so how much will they take off?

l do not think you will actually

For annual,monthly, & weekly rates, look at listentotaxman. As it is ur first job, u will have a backlog of unused tax allowance to 6th April 2008. You can earn £6035 a year before paying tax. It will depend when u start, but it is unlikely u will pay tax before next April on that level for part of a tax year.
Beware, though, after next 6th April. Depending what tax allowances that will shortly be announced for next year, u will probably start paying tax then. So do not get too used to a high-spending lifestyle!
National Insurance contributions will still be deducted. That is done on a pay-day basis, irrespective of earnings for rest of year.