How Can a 13 Year Old start his own computer repair/customize and get made compuer business?

He will need an older, at least 18 years old partner, so clients could trust the company.

There are two basic methods:

1) Create a business plan and follow it

2) Just do it.

Assuming you'll be a sole proprietor and working out of your house, you're biggest challenge aside from getting some dog computers that are difficult to fix, or past their prime, is finding clients. Unless you advertise, you'll find clients mostly by referral.

In any case, if that's where your passion lies, then keep doing it, and keep putting yourself out there on social media and other forums using your real name to answer questions and demonstrate your abilities. Prospective customers (or employers) are likely to Google you, and it helps for them to see corroborating evidence that you've been doing computer repair, support, etc., and that some of your answers were spot-on and helped resolve someone's problems. Guard that publicity closely, and avoid controversy, politics, flaming people, or anything negative that would put someone off from doing business with you.