It's actually 20% of the profit, but there are special rules for the first year to cover any differences between your accounting year and the official fiscal year, so you are best advised to consult an accountant (which I am not !). The rates can be found on the HMRC website:
http://www.hmrc.gov.uk/rates/it.htm
If you file your accounts online, the tax will be worked out automatically for you, but please see an accountant as he will advise you as to what you can claim, and things like deductions for personal use of telephone etc.