You can buy it at a store like wallmart or gamestop, or you can link your credit card to your account and pay for it that way.
At your local game store. So, yes.
it enables you to play online with other people
You can buy 1 month, 3 months or 12 month cards. 1 month cards are $6.99, 3 month cards are $19.99, and 12 month cards are $49.99. You can also register for Xbox live on the Xbox or Xbox360. Hope...
you get the xbox live gold membership from game shop eg. (ebgames,game,jb-hifi) etc.