First steps to register as self-employed

How and when to register:
When you become self-employed you must register for Income Tax and National Insurance purposes with HM Revenue & Customs (HMRC).
There are three ways that you can register:

•online using the email links below (recommended for most)
•phone - by calling the Newly Self-employed Helpline on 0845 915 4515 or 0191 213 5000. Lines are open from 8.00 am to 8.00 pm Monday to Friday and 8.00 am to 4.00 pm Saturday and Sunday (closed Bank Holidays).
•post - by downloading and completing form CWF1
If you are self-employed in a partnership, each of the partners must register separately.

You will need to provide the information listed below:

•name
•address
•National Insurance number
•date of birth
•contact telephone number
•contact email address
•the date your self-employment commenced
•the nature of your business
•your business address
•your business telephone number
•Unique Taxpayer Reference (UTR) if you were previously within Self Assessment
•the business's UTR if you are joining an existing partnership
•where relevant, the full name(s) and date(s) of birth of your business partner(s)

As well as registering as self-employed you’ll need to arrange to pay Class 2 National Insurance contributions, unless you expect your earnings to be very low. The most convenient way to pay Class 2 National Insurance contributions is by Direct Debit. If you are unable to pay by Direct Debit, quarterly bills will be issued in January, April, July and October.

source: www.hmrc.gov.uk/selfemployed/register-selfemp.htm