​How to start this business

So I’ve been talking to people around my college, they were wondering how much it would cost for me to build them a pc, I love building pcs and yes I know it’s already a thing but as a local they can ask away pay cash or online, custom build with a live physical consult fully on how they wish it to be I could do a 10 mile range from my address I already have around 15 people wishing me to put them together the system they wish I just need to know if it would be smart as a 17 year old and where would I start with building a website. Yes I’m 17 but I gully understand tax laws and if I make under around 12000 gbp then I am below a tax bracket and I should be fine to continue making most profit, yes I understand all the bits and bobs of online transfers but all I’m looking for is how to build the website and if it’s a decent idea for me at 17