Ok, a few hints...but you should go online and review the policies...even yahoo has a lot of advice on credit cards and scoring...

-It is okay to use one card and not others but you should also know that companies will not increase your credit by a large amount if you do not use it. If you use it more and pay on time, they are likely to increase your credit limit....higher the limit, higher the score.
-Spend within limit-less than 25% of your total limit and do not miss out on payments...if you do miss, talk to the company and ask them to consider it for one time....before they put it in your credit history.
-You can close one card and get a new one with better options but remember that every time your credit history is pulled, it will have some negatives on your account. So, frequent applications to credit cards are not advisable.
-I would advise you to use different cards every other month or so...this will help you increase credit limit on those cards...