Lives in my car for 2 months in San Francisco. Long story really. But basically this is what I did:
Got 24 hour gym membership to shower/shave and stay in relatively good physical fitness. Cost about $40 or so a month only on a month to month membership.
Got job at Burger King to support myself. Got lucky and worked morning shifts. Had evenings to myself
Got ups store mailbox which gives you a real address, different from po box which isn't a real address. This is important when applying for jobs and not appearing homeless.
Generally had to buy everything pre made since I didn't have way to cook food. Had no refrigeration either, of course. So a lot of deli sandwiches or canned foods for dinner. Sometimes BBQ'd in a park for some good warm food on the cheap
After two months saved up enough money to get my car fixed and have gas to drive back home to Florida.
I don't know your situation but living in a car is much better than living on the street. It is very scary though and I wouldn't wish it on my worst enemy. If you can find any way to not be homeless even if it means living with your parents or paying friends to sleep on their couch please do it. I was dumb, ruined the relationship I had with my parents, and had no one to turn to in my situation. Don't be stubborn if you can help it and have someone who can help you.