Hi mate,
You have a legal obligation to inform the Inland Revenue that you are self employed within 3 months of starting to trade. This applies even if you are employed as well by another company. Whilst you are at it tell them you want to be on the CIS scheme, this will get you a Unique Tax Reference number (UTR) you will need this if you work some builders etc who pay you on this scheme, they pay you 100% of your materials and deduct 22% of the labour and pay it to the taxman on your behalf.
Speak to an Accountant, i can highly recommend one PM me if you are interested.
Keep receipts for everything, tools, materials, courses, hotels, meals, professional fees (part P etc), and anything else.
Keep all household bills, bank statements, phone bills, mortgage statements,and council tax bills, because if you trade from home you can claim part of these costs against tax.
Vehicle costs are optional, you can claim 40p per mile for business use, or you can claim the actual costs like MOT, Insurance etc, an accountant will advise you the best method based on your circumstances.
Thats about all i can add, but take up some of the good advice in the other posts.
Cheers..........Howard