What point will you have enough credit history to sign for your own car?

I went to a dealership to buy a car at which point they told me I would need a co signer because my credit score is at 625. My mom cant sign for me because her credit is at 5 something so it wont help so I need to get a cheaper car. let me ask at what point will I be able to sign for my own car, and what price range should I be looking at to buy a car. Also what things like cell phone or paying of credit card bills can I do to raise my credit or even get a car and make payments that will help me raise my credit enough to sign for a $25,000 car by june my self?