Do you have to be 21 to rent a car or are there places you dont have to be with a credit card?

Actually, you have to be 25 to rent a car. Some companies will allow you to rent if you're under 25, but with additional fees and holding a credit card until return. Some companies won't rent to anyone under 25 at all, though.

Of course, this only applies to Chicago/United States, as that's the section you've asked in.

Anyway... pick a company and go to their website. Each company has a slightly different policy, and most are outlined on their respective websites.

No you have to be at least 18 but they will charge you more and a high deposit. I think they will make you put a credit card on the account as well!

ive rented cars before and never again. with the amount they charge you would be better off to buy a $500 car and sell it off again after a week.

I believe you have to be 23 or 25 to rent a car, and it is possible to rent one with no credit card, but you will have to put down a pretty large security deposit, I would say to expect to put down around $500.
You can always get somebody with a credit card to rent the car for you and put you on the drivers list, but they would have to trust you to not f**k up the car and their credit. Those are really you're only options if you need to rent a car.