Going Self employed

I'm just looking into the possibility of becoming self employed. I am looking at running a small homeboarding business from home, (it wont be the households main income, my partner works full time and pays the bills ) I want to earn a little money whilst I'm at home

my question is surrounding the rules of being self employed as I don't want to start it off wrong,

I know I need to register with HMRC , that's a given
my question is around National Insurance mainly
I anticipate earnings to be around £7,000 per year so under the tax threshold but want to make sure I Pay the right National Insurance
and what things can I claim back as business costs
sorry if these questions are easy for some , but want to make sure I do this right from the beginning, any help would be appreciated