Hw does trading in ur car work? please help?

So my dad who has really great credit got me a bmw. (I have horrible credit because some one stole my identy) the bmw is to expensive upkeep. Tires are $400 gas is 5$ a gallon. To expensive. I still own 9000 on my car and the kelley blue book value is 13875 -in fair condition. I want to trade it in for another car. How does it work what price am I looking at? Ps my dad has on many occasions walked out with s brand new car with no money down..