See an accountant before you start. Find out the system he recommends for bookkeeping, and use it. Enter your books regularly. File receipts and documents so you can find them. Get the right insurance for your needs.

A good accountant will advise you on whether to register for VAT or not (usually not) and whether to form a company (not always an advantage and increases paperwork).

If you have any form of partnership particularly friends/family have a proper partnership agreement drawn up.

Good luck.

Stu