Bad credit doesn't always mean you can't get a credit card. Lenders have their own criteria for applications, and they'll consider more than a credit score when deciding whether to lend to someone. Having a poor score doesn't guarantee your application is turned down, but it can limit the number and quality of offers you get.
If you use a credit card responsibly, it can help you improve your credit score. That means sticking below your credit limit and making your minimum payments on time. Not doing so could harm your credit rating.
To find out if you'll be accepted for a Capital One credit card before you apply, try QuickCheck.