When I was 18, last year, (damn Im young), I bought my gf a car, well she's paying for it but the loan is all in my name, but anyways.
The car was 3500 at a local ford dealer, me being 18, i'd had a CC for about 4 months and my cell phone bill in my name for about 6 months, other than that no credit. But I still qualified for a 4% interest rate, so your doing something wrong if between you AND your mom you cant do better than that.
Whew, sry was so long.
But good luck, I like paying cash for me cars, main reason why I haven't bought a car for more than 6500, lol, but its still sweet.
Matt