The basics of all bookkeeping:
1. Record all money you receive.
2. Record all money you spend on business related things and keep invoices to prove it.
Provided you keep these essential records, all subsequent calculations for tax etc will be simple arithmetic.
When you first start up, you need to keep things as simple as possible, there's plenty of time to play around with fancy computer programs etc when you've got the hard slog of setting up your business well under way.