They will quote an hourly rate for their time and tell the customer that they work on a time and materials basis. That means X many dollars per hour of time, plus the cost of parts and supplies. Most shops also charge for research time, shipping of parts, recycling, rags and so forth.
You must be 18 years old to get a credit card by yourself. While you can legally get a credit card as young as age 18, the law requires you have your own steady source of income to qualify for a credit card. Unfortunately, a monthly allowance from your parents doesn't count. It's for your own good.
If you believe someone has attempted to create a credit history based on your child contact the bureau's ASAP. Anyone younger than 18 can not be legally held responsible for a debt. I would not worry about it before 18. Once you turn 18 I would go get three bank credit cards and go from there.