AsherGomez

Building PCs is a niche thing. Not many people need that done and the ones that do usually go with people that already have a reputation behind them. 
Linus did the "Summer Job" 2 part series for making money for PCs. 
iirc, most US employers aren't allowed to hire you at 15, so if you did any work outside of things like this it would have to be under the table. 

As someone who does that on the side, I'll offer my 2 cents (and a bit of context about my situation):
First, about what I do: I have a PC side business. I do not actively advertise or otherwise seek clients. I am a full-time student, or full-time employed at all times, and don't have time to fully into the PC business. So it's just a side thing. But here's how i do it, and some tips/info about how the market is: If you want to build "normal" PCs for people to use at home for like, MS office and stuff, don't expect to make money. Frankly, best buy will get people similar or even better performance than you can due to the competitive mainstream market and their volume discounts at the manufacturing and selling levels. You might be able to convince people that the serviceability, upgradeability, and modularity of a custom solution is good, but you still won't make enough $$ for it to be worth more than "getting your name out there", helping a friend out, or like me, just having fun building PCs When it comes to gaming PCs, you can actually make a small amount of money. Don't expect more than 100-150 bucks profit though, even for higher end rigs. What that is per hour depends on how fast you are. I can build a standard gaming PC in an hour, but I've been doing this a long time. Remember, the lower-end stuff is, the lower your margins will be If you can find people who want custom watercooled builds, or modded cases, those will get you more profit. Hundreds, plural (especially watercooling). Those are hard to find, and it means often promising some sort of availability to help with maintenance, etc if you want to get some customers. But it is one of the better ways to actually get good profit. Case modding and such is also a good method, because you're providing something many others, even OEMs can't do. i.e. I have built briefcase-sized watercooled gaming PCs by modding some ITX cases that people just can't do without tools, confidence, and knowledge. That adds value to your work. The last, and IMO most profitable category, is business. I have built a few CAD workstations for engineering firms, and those are good profit. You can make 500+ bucks on a machine that is not physically more complex than a gaming PC. If you can find people who need those, that really helps. If you can build relationships with those compaines, that's even better. I have been asked to return for upgrades, more PCs, and even networking/LDAP setup and related IT tasks. That's key. I can't help you much with marketing as I don't do it. Everything I do is from personal connections since I don't really have time to dive fully into this. But hopefully that gives you some perspective and ideas on how things will work. One note I have for you, specifically, is that you will have more issues than I do with actually going through the ordering process with customers (probably) because you're not an adult and don't have credit cards with high limits. I can order the parts for a few CAD workstations before I get paid, assemble, test, and deliver, and be okay waiting to be paid until delivery. You may need help from parents, or perhaps ask for cash up front from customers to do this. That may be less than desirable for some, so keep in mind ways to try and get around that if you can. Also I advise getting an account with square or freshbooks (shameless LTT sponsor plug) or another payment processor so you can take credit cards if people prefer that (which some do).
 
Good luck, and if you have any questions, feel free to @ me or PM if it's more personal.

You can find tens of threads on forum asking the same thing. Reality: Repairing and upgrading, cleaning etc. is much better way to start than building.
 
There are few things to consider here. Whose money is going to be used, and who is responsible for results or if anything goes wrong. If you are under your country's/state's age limit for responsibility of loss, then responsibility would drop onto your parents insurance. If someone would hire you, then they would be responsible of your actions when you are at work. So, if customer gives you $1000 for building them gaming PC, probably up to some spec you've agreed to. If that PC fails set spec, then you have just wasted customers money and any fixes/compensation would come from your own pocket. Same goes to any warranty. You are their OEM and you are responsible of handling anything that you might have caused. Not hardware issues, but loose cables or something gone wrong with OS install. Thats your time (and money) for fixing it. More the money involved, more customer expects you to perform.
 
So back to why repairing, cleaning and upgrading is better. Because thats rather easy to do. You probably already do it to your family and relatives. Set $30 per gig rate and you are already ahead of shops (which have $50-70/h rate). Update Windows and software, run some cleaning tools. Easy. Consult on hardware upgrade, install it and make sure everything works. Clean some dusty mess, do backups and stuff like that. When you get some customers who can vouch for you, then you might get bigger deal, like building gaming PC.