A.: You have made an understatement when you say you haven't planned well. Having a car payment in excess of a week's pay is absolutely unforgivable. You should consider replacing it with a far less expensive vehicle.
I don't know where the money would come from to invest in a 401(k). Given your current situation, if your income doesn't improve, I don't see you being able to retire for the next 15 years or more. Getting the zero-interest credit card is not the answer.
I wish there was something more cheerful that I could pass on to you, but the reality is that Social Security will not provide enough for you to live on, and you have no savings. Good luck.