How do I quickly buy a car?

So im 16, and my mom's making me buy my own car. I have a job and I make from the high 300's-400 a month. Ever since I got a job, my mom makes me pay for everything else too such as clothes, movie tickets, boyfriend's garter, Halloween costume, etc. I know I shouldn't be spending any extra money but it's so hard not to. I really want to buy a car fast because I have a lot of extra curricular activities dealing with school, sports, and clubs. Not to mention it would be nice not to have to ask for rides everywhere. The cars im looking at range from 7,000-10,000 dollars and my mom takes half of my paychecks into my savings and that's just normal savings for later on in life; not for a car or anything. So that makes it really hard on me too. I need a good strategy or something...please help me!! :(