If you are self employed, you would register for Self Assessment as self employed.
HMRC would send you a 10 digit Unique Taxpayer Reference number (UTR) for subitting a tax return, so that you can pay tax and national insurance.
The tool here:
Check employment status for tax
Will help you determine if you are employed or self employed.
If you are employed, your employer should be deducting tax and Nationa Insurance.