A self-assessment is a horrid affair which becomes more difficult (and costly) the later you leave it. Avoid these costly and common mistakes (and TAXO'D the taxman) when filing your tax return
If you work as freelancer / are self-employed then you will need to apply for a UTR. Which is a number assigned by HMRC to be able to complete your Self-Assessment Tax Return.
Thinking about working for yourself? Then you will need an understanding of the taxes you will pay to HMRC. Here is the TAXO'D basic guide to: Understanding Income Tax for Freelancers and the Self Employed.