Does the car dealer give you a longer time to pay off a car or shorter time? Some people get financed for 36 months, 48 months or something more. So if my credit is bad, would I be more likely to be financed over a longer period of time?

you would be better off not to buy a car or have extended payment plans
you sound like you are easy pickings for a loan shark
Work at rebuilding your credit - and do it in a consistent and honest manner
Buy a cheap car if you have to
Get a bank account
Maintain a balance
Take out small loans and repay them
build a credit record