and buy a sim card to your store for ativation?
Note that Mobilicity's Nexus S is not locked. All you need to do is change the SIM card.
This is actually a bit of a confusing answer as it requires some technical knowledge about phones and how they work. If you have a phone that is not sold by Mobilicity and you’d like to activate it, you have to check the frequency of your device and ensure it’s compatible with the Mobilicity network. Mobilicity’s network uses a frequency called AWS, with bands 1700/2100. This is a fairly new frequency for phone carriers to use and there are only two other carriers in North America that use it. Additionally, your phone will need be to be “Unlocked” and you’ll need to purchase a Mobilicity sim card. If in doubt of whether your phone is compatible, visit a Mobilicity store for assistance. source