Are you carrying a balance on any of your cards - I assume so since you mention your debit to credit ratio. If you do (or ever do) then getting into the mileage game at this point in time is not for you.

As others have said, with a mid 600 credit score, you must have made some errors in the past. Concentrate on making sure you get all your credit cards paid off and making all payments on time. Use the credit card you have to make all your purchase and pay it off PRIOR to the bill closing date. That will keep your debit to credit ratio in the lower percentages.