A decent kit will run over $100. As said before, you can get started for less, depending on what you have at home already.
It is legal to brew 100 gallons for a single person, or up to 200 gallons for a household with more than one adult.
It is illegal to sell your homebrew in any fashion, including barter (trading for favors). But you are likely to be able to give a certain amount away. Check your state laws regarding the gifting of beer and wine.
The process of selling homebrew (called craft brew when you get the license to sell it) is a difficult one. First you have to obtain a proper license, which is cheapest, I think, if you know someone who operates a bar. They can purchase a license for their establishment, and you can brew there ONLY. On top of that there are regulations to upload, inspections, quality control, records keeping, unexpected expenses, etc. The list goes on and on. It is VERY MUCH more work and cost to brew under a license than for brewing at home.
Brewing is a very fun and rewarding hobby, but I feel like I wouldn't be doing you a favor if I didn't repeat something: There is a very good chance that at your age you will be tempted to give beer to friend who are underage. Don't do it. Bad things can happen, even outside of a vehicle, and we won't even mention drinking and driving, which ought to be verboten in your mind already.
If you are just starting out, maybe look for a homebrew club in the area. I bet there are lots of homebrewer who would love to pass on their experience and you could get firsthand help getting started. In the HAM radio world these people are called Elmers. I'm sooo glad we don't call them that in brewing!