firstly you need to register that you are self employed and have 3 months of trading to do that.
keep all your receipts, buy a pile of envelopes and write down the month on each of them and pop them in their for easy reference.
yes as collin says, you an offset everything to do with starting up the business and claim it back, you may even get a tax rebate at the end of your financial year as you won't be expected to have much earnings the first year.
give tax office a ring and they will send someone out to your premises or your home and they will give you relevant paperwork to calculate your tax, and also give you advice on how to set up and any new laws and legistlation thats happening.
set up a seperate bank account or post office account and get a standing order set up to pay a certain amount of money into it each month for a tax bill, say £75 a month. If you have any surprises at the end of the tax year you will be prepared for it.
write down everything that you spend, even if its 25p for an envelope, 36p for a stamp.
type up an excel sheet with categories and listings that are applicable to you,
eg
week 1 week 2 week 3 week 4 week 5
car expenses
petrol
repairs
insurance
tax
MOT
parking
if you do it on excel it does your calculations for you