It's my understanding you can't open a business Until your 18. However you might be able to fix computers on a word of mouth referral. Most people are desperate to find someone cheep who can fix what ever is going south on their north bound computer.
Be careful not to bite off more than you can chew. Once your reputation is damaged it's really hard to repair it. Also be sure that both sides know exactly what your being asked to do and how much your charging to do it. There will be those who will want your services but refuse to pay you and there are those who will claim you broke their do dad when in truth you did not even touch it. be sure you have a ready answer and proof.