Great that you want to start your own business but I would suggest something different as computers are dropping in value and use that your margins would be sa small that your business would very likely struggle from the start and possibly even fail right away. So your money would disappear quickly. I would suggest creating a small specially built system for a very specific market niche. What that specific niche is you will need to determine by your area and what people are buying. Say it was a gaming PC computer you found people in your area wanting. What you would need to do is see if you can effectively build a gaming system that is cheaper and has better specs than the big guys (Dell, etc). If you can t or you only make $20 it is not worth it. Find something else to do. I hope this helps you think it through. Good luck
if you have knowledge i would build 1, and maybe sell it around your town, or go and put it up on ebay. If u see people find it nice then make a website, and sell there. And to go abouve and beyond after a while build a shop