A credit score of 600 is not very bad but a credit score 600 or under is considered to be a pretty poor credit score which means it can be hard to find credit cards that you can get approved for.
But you might have some luck getting one of these credit cards.
Capital One® QuicksilverOne® Cash Rewards Credit Card
Credit One Bank® Platinum Visa® with Cash Back Rewards
Or if you cannot get those you may need to go with a Capital One Secured Credit card or Discover Secured Credit card where you put up a security deposit and then get the money back after awhile of using the credit card properly and paying the payments properly.
Then the secured credit card will build your credit to where you can get a regular non secured credit card and continue to build up your credit.