Microsoft points are a form of currency for the Xbox or Games for Windows. The conversion factor is 80 points per dollar. You have a couple options to buy these points:

1.You can buy a card with different point values at a convince stores or a GameStop. Then once on your Xbox or computer you can Redeem your code on the back of the card.
2.When on the Xbox, if you go into the marketplace you can add points from there, but you can only buy it if you have a credit card linked to your account.
3. You can also buy Microsoft points from the Xbox website.
4. If you want Micosoft points for Games for Windows. Open the client and click the "Marketplace" button. Then follow the instructions to redeem your points.