Welcome to the area !
Your info you request is at this site...
Window Phone App Redemption
You need your live account AND you Serial number of your phone (IEMI or EMI). This will give you a $25 credit that will go in your wallet on your phone, so when you buy an app you just select Microsoft Credit and it goes towards that. $25 in free apps...
BTW, With just browsing or searching you could of found this info in less time than it took for you to write your post. There is a few threads on the subject. Next time, please take a second to browse around before posting a new topic.