This is a two-step process out of which the first one is more important. You have to purchase a new Windows Phone until the end of this year — eligibility starts from November 2 — to be able to benefit from this offer.
Microsoft is granting $25 Prepaid App Cards for new Windows Phone owners, granted they submit all the info no later than February 14, 2012, so that they can spend the money on new apps and games from the Marketplace. Chances are all you apps are already there for Windows Phone as it hit the 40,000 app milestone, so $25 will at least grant you a dozen paid applications (or even more). Follow the source link to claim your card if you are eligible.