When you make the decision to go self-employed you need to contact H M Revenue & Customs and fill in the form CWF1 online. You will be given 3 months from the date you first start trading to complete this, however, if you are late, HMRC could issue a penalty. You will be subject to Class 2 National insurance which will be collected via your self-assessment tax return, as will class 4 (where necessary).
Working for yourself and becoming self-employed can be very rewarding, however, it is always advisable to seek professional help first.
If you are starting out as a freelancer or sole trader the processes of becoming self-employed are straight forward, however, if you are considering trading as a Limited Company my advice would be to seek professional help as this is more complex and requires a detailed setup process.
That’s the basics of how to register as self-employed, there are other issues to consider such as banking & financing, marketing & promotion.