Firstly you need to keep a track of all your income and outgoings.
Its not that difficult, if sole trader you can simply put all your receipts in an envelope for each month (put the receipts in as soon as you get them or you will leave them all over the place) do this for each month April to March (tax year 6th April 2012 to 5th April 2013).
Keep a record of payments received, again do it each week to keep it simple.
If you pass all that to an accountant (or if its simple just a normal book keeper - no need for chartered accountants at this stage, they charge a lot more) at the end of the tax year, they will do it all for you, it shouldnt cost that much. This way you know its been done right and your not going to worry about the tax man chasing you up, if they do, you have all the records to hand. You are supposed to keep this info for a minimum of 6 years - just store it in a box in the loft or somewhere safe should you need it - done... do the same the year after, so on and so on....
If your business gets bigger and you have more complex affairs then it maybe that you need to sit down with an accountant to sort out what they want from you each year, some will agree a fee so you know what to expect, again this way you know its all done and you can concentrate on building your business up
Only my opinion, but i find it best this way, as i know its all done and i dont have to mess around trying to get it right and working out what tax rules have changed, what i can and cant claim for etc etc. A decent accountant will save you more than what they cost.
Cheers
Steve