Microsoft points, pre paid cards, and gold subscriptions.

Microsoft® Points is a stored value system that allows consumers to purchase value for their account and use that value to redeem digital content. This service is supported by Zune™, Xbox LIVE®, Games for Windows – LIVE, and other Microsoft properties and partners. Definition provided by xbox.com

There are two ways to get these points and two ways to redeem them, on is through buying either a 1600 point pre paid card or a 4000 point one. You can redeem the codes either on the 360 in the marketplace or on xbox.com in your account management. The second way is by credit card on either the 360 or xbox.com. The point purchases on there include 500, 1000, 2000, and 5000.

Gold subscriptions can be purchased the exact same way, the prices list as:
$7.99 for a month or or 26.6 cents a day
$19.99 for three months or 22.2 cents a day
$49.99 for credit card purchase of one year or 13.6 cents a day
$49.99 for prepaid card purchase of one year + one month or 12.6 cents a day
Any purchases made by CC are set to automatically renew unless the customer calls into cancel it per section 6.6 of XBL terms of use.

Prepaid cards- If the card you by is still a scratch off card and not a peel off card make sure you scratch lightly or else you might destroy the numbers on the card. From that point xbox support cannot help you, if you cant get a refund from the place you bought your best option is to send an email to XBL support and microsoft support with a copy of the card and a discription of what happened and they MIGHT help you. Other then that just try to guess the missing numbers.