Self employed on top of full time job

ForestBluebells

I work full time and use my personal allowance in that job. I just started some freelance work last month and when I get paid I’m aware I need to keep aside some of the money for tax.

Do I only need to register as self employed once I earn £1000 as that seems to be the allowance before needing to register? Or do I register now anyway? It’s just I don’t know how long I will have the project for so I’m hoping to earn around £3k this year but if the project gets dropped then I won’t.

Also do I just keep 20% aside for tax? I don’t think I need to pay NI contributions as I won’t earn enough outside my full time job to pay this? I also have a student loan but I should have this paid off within the next 18 month or less and I will move to direct debit payment of this soon too. Or do I still need to factor in payments towards this?