What you have to do, besides transferring your number over to Sprint, is to buy a smartphone, whichever you fancy from the carrier's lineup, and sign a 2 year contract. If smartphones are not your cup of tea, there is no need to worry. Choosing a budget handset or a feature phone will get you $50 in service credit, which can always come in handy. It is very important to register your phone within 72 hours after activation. Further details can be found on Sprint's website and we strongly recommend reading them if you plan on taking advantage of the promo.
source: Sprint via Engadget Mobile