If you are going to be working self employed then you need to register as such with HMRC and also arrange to pay your National Insurance contributions.
You have 3 months to register from when you commence your business.
HMRC do run courses in all sorts of things to help you keep your accounts and fill out a tax return, they are usually very helpful when you contact them.
There have been a lot of threads on here about tax and the various problems that can occur, so if you put tax into the advanced search on here you will also get a lot of info.