you should first, try and sell it for what you owe if you can not find a buyer for the price you owe then I suggest selling it at a loss. Always remember it is better to lose money then screw up your credit.
Get rid of it for good money and get used to public transport. Get a quality bicycle too. Living in a modern city and driving around with a bicycle is a viable option for anyone. That way you're combining exercise and doing good for the environment as well!
Call the bank that you have the loan from and tell them or try going to a car lot where they have less expensive cars. You can also try refinancing for longer terms and smaller payment. i. e 3oo for 36 months to 150 for 72 months.
If you can't afford your car anymore you should sell it and use public transportation. Maybe you can find someone who can take you where you need to go, such as work, and pay them a small fee to help cover costs.
Your best bet is to sell the car yourself. Try Craigslist or other sites like it to post your car. After your car has been sold take out a personal loan to cover the difference. This will be much easier to manage than an entire car that costs too much.