I trained as a mechanic in the 70's and always wanted to set up my own business. When I looked the cost was huge to do it right. A mate trained as a plumber and bought a blow torch and a van and made a good living without being too taxed work wise. I wish I had done the same.
If I was going to do anything now plumber would be my shout. To get Gas safe registered you need to train and pay but I know a couple of lads who never did any of that and still make a fortune as most call outs are toilets and sink leaks where you need no special qualifications.
I know another who will install boilers etc. but gets everything signed off and checked by another gas safe engineer, there is just so much work about.
It does depend on where you live as to what you can charge, but when water is pouring through a ceiling cost is the last thing on peoples mind.
It is a low set up cost, chose what work you take, a lot is paid cash so little in way of debtors.
Some people are not wired to work for themselves, they need to be driven, well motivated and able to manage the ups and downs, like if you are ill, where does the money come from.
At your age if you are going to do anything do it now in a few more years you get too comfortable with a good wage coming in and the security that gives.