You need to keep accurate records in order to restrict the taxmans ability to screw every penny out of you that he possibly can.
A healthy accounts system also assists you if you deal with banks for loans etc
In order to satisfy these statements,basically you need to have a record of what monies you recieve and what monies you paid that were relevant to operating your buisness
These two can be sub divided to show more detail,eg a record of invoices issued to customers,bank account details,material expenditure,motor vehicle running costs and purchases of tools,workwear and anything else that is relevant
The more expenditure that you show the lower the tax liability at the years end,so be meticulous and accurate with these details,cash jobs tend to have less impact on peoples bookeeping for some reason
My advise would be to book ypourself a visit to an accountant who will guide you and sort out the jumbled mess at the years end(and he's deductable as well)
Some who are confident and adept at the paperwork side of things do it all themselves,but you are new to the operating,so may not be wise for you to do so