Just use them the next time you need to make a purchase (groceries, gas, games, internet, etc). When it comes to improving your score, credit utilization ratio is important. This is the amount of credit you use relative to your credit limit. It's recommended to keep your credit utilization ratio 10 percent or lower. So if you have a $1,000 credit limit, don't spend anymore than $100 dollars per billing cycle and you'll be good. If you don't spend that much, perhaps you can ask your carrier if it'd be possible to lower your credit limit to like $500.
Just be sure to pay off all your credit card balances in full each month (don't default) and your credit should improve. Also, don't close any old accounts even if you don't use them that often b/c the longer you maintain a credit card account, the higher your credit score will be. There's a lot of information about what goes into a credit score here
http://www.is650agoodcreditscore.com.
