I am 18 years old. How do I get a car loan for $3,000?
A better tack would be to ask why you need such a loan? If you MUST have a vehicle, you are probably stuck asking friends or family unless you are going to pay a pretty high price for borrowing the cash.
Is $3000 out of your range to save up?
If you can possibly get away without having a vehicle at this time, it would benefit you more to build your credit (search this site for how) and borrow only when you need.
If you have no credit, then whatever money you borrow, from whatever source will have a high cost. It would be better to do it the traditional way (again, if you can wait)