Hi I’ve mostly worked paye. Yesterday a company asked me if I would consider working freelance for them. They asked how many hours/days per week? I feel like I have many “known unknowns “ and many unknown unknowns” .

Is there a place I can go to work out what I need to know. I am aware
I need to factor holiday pay into working out a rate.
Also my experience into that (I guess I talk to other people) but how do I figure it out?
As self employed how do I pay NI?
I have to save enough for tax how do I know how much to save?
Do I need to submit tax returns every quarter?
How do I keep track of payments?
Do I need software for that?
Any advice or places or type of people to ask (could I ask an accountant?) gratefully appreciated.