Fitness tips for someone who wants to become a firefighter?

I'll keep this short. I want to become a firefighter, and I'm going to apply when I'm 18, which is in four months. Obviously firefighters need to be in peak physical fitness, which is something I am not. I'm a 17 year old who sits around all day playing Xbox and watching movies. I'm not in eduction, at the moment and I haven't currently got a job, so I'm not active at all. Also, as I live with my parents I can't really change my diet drastically, because they won't go out a buy lots of healthy stuff just for me. So, what's a good way I can drop my belly fat, and generally get fit enough to pass the fitness tests for firefighter training, in about 6 months, or faster if possible? Thank you very much!