That's OK! It's a massively long thread I know, but basically you just need to keep records of what comes in and what goes out (this is any money you spend on things you need to run your business; condoms, hosiery, taxi fares, whatever) and then at the end of the tax year you add up what you've earned and take off what you've spent. This is your profit, and what you pay tax on after your personal allowance which is the amount you can earn before you start paying tax.
So if you earn £40K and spend £15K, your profit is £25K. The personal allowance this year is £12,570, so you'll be paying tax on the remaining £12430 (and also National Insurance, but that's a story for another day)