Hi,
This is not something that can be advised on via this forum. We need specific information about you and your circumstances before this can happen.
If you are wanting to register as Self employed you can look at the information available at GOV.UK
If you need to declare tax but not be registered as self-employed you will still need to register for self assessment.
Check how to register for Self Assessment
If you are still unsure you will need to contact us directly via telephone or in writing.
Thank you.