Hi
Before opening my beauty salon I was an Accountant for 10 years, not up to date with current tax law but you need to register your business in a certain amount of time, which I can't remember but what I do remember is that if you register late you get a £100 fine. so please check this out.
You also need to register to pay your NIC contributions which is roughly £2 per week.
I would recommend getting an accountant which can be costly but meet with them and get them to design an excel spreedsheet or something for you to enter your income & expenses on to minimise cost. So all they need to do is enter totals into their system, they probably wont offer this but they will do if you ask.
Also take into consideration that if you make a certain amount a year you then need to register for VAT, this is over £65,000 per year, again can't remember the exact amount.
The Inland revenue aren't as scary as people think and they will give you all the help and advice they can.
Hope this helps