Xbox Live is an online game service that allows players to compete on the "team play" network Xbox Live allows users to: play games online; download games (through Xbox Live Arcade) and game demos; purchase and stream music, television programs

Check XBOX Package if it is written on it NTSC then you have to buy United States Card
 If it is written on it PAL then you have to buy EU Card
You can make sure(in case you lost the XBOX Package) on the link below to make sure which card you should buy Click Here
After you sign in you will figure out your account type, and the matching card to purchase.
Log in to your Xbox Live account.
Click on “Redeem Code” and enter your card details, then click confirm.