I work at a local PC repair shop, and my boss taught me a lot about how he runs his business. It's a lot harder than it sounds, it's definitely risky. He's been in the business 17+ years so he managed to be one of the first in the market(thus gaining a loyal customer base), but you'll have trouble competing with other big names such as the geek squad or other companies that are well known and trusted. Getting started will be the hardest part as customers would be worried to trust the new guy.
Big name manufacturers(Dell, HP, Lenovo, etc) all get massive discounts on hardware and software because they buy them in massive quantities. The hardware is a lot more expensive for you since you need to buy from a reseller. For example, they get copies of windows for less than $20, whereas you would need to pay over $100. Building computers to sell isn't really viable, because you can't really get good deals on components that would allow you to turn a profit.
While you might be able to turn a profit selling/repairing computers, it's still risky. Running a physical store costs money for rent(my boss says his shop costs $5000 a month to rent, and its a small building too), and so does advertising, compliance fees, hiring an accountant to cover legal things like taxes, etc. Just breaking even is difficult, and don't forget that you still need to profit to support yourself.
I would advise applying at a local PC repair shop to learn what it's like. You'll run into all kinds of things you'll never expect:
Think windows 98 is obsolete and you won't need to learn it? Nope, customers come in with it all the time. Small businesses are actually one of our largest clients, and they need to maintain their old operating systems, as their software is only compatible with older versions of windows.
Repairing customer PCs is also a lot harder than working on your own, custom built PC. Laptops and all in one workstations are huge nowadays, and they don't follow the ATX form factor. You'll need to learn how to pay attention to details, refer to documentation on how to disassemble different models, and be good with your hands. I'd say maybe 60-70% of our PCs we get are laptops. You can't just deny every laptop with a hardware problem either, or you're losing business. My boss knows how to fix broken solders, replace transistor caps on the motherboard, completely disassemble different laptops to replace things like fans/screens, and all sorts of complicated mechanical fixes. Without knowledge of this, you will likely fall behind your competition.
You also need to develop a clear policy to avoid legal issues. We frequently have customers that bring their computers in, but then disappear and never reply to our phone calls. Do you just hold onto their computers for all eternity? No, you'll run out of space, so you need a policy. You can't just throw them away or sell them without informing the customer of the policy, as they'll take it as theft and potentially sue you.
What about your policy for pricing? Do you charge for Diagnostics? What happens when they find out how much it costs after you fix it, but don't want to pay it? Do you wipe the HDD, reformat it, and resell it, if they don't agree to pay the costs?
There's a lot of planning that goes into running a business, I wouldn't suggest starting one until you have more experience, as its quite expensive to startup, and could put you in debt.