You said you're great with computers, how so? Do you know howto code, design, fix computers, or? Being more specific can help you get a better answers.
In any case, check out sites like Fiverr.com,Upwork, Elance, Freelancer, etc. You can build a profile and post what you're skills are. Then, people can hire you to do small projects for them. You can even post on Craiglist, just be careful who you deal with. I believe all these sites allow minors tosignupas well, but if not maybe ask your parents tosignup.
Computer skills are highly sought out in most areas and if you're good it doesn't matter how old you are, so build your portfolio and an online presence to show case what you can do. You might even get lucky and be found by someone looking for your skills.