How can I lock my stolen iPhone? Last night my iPhone 5 (32 GB) was stolen.

How can I lock it so that the person won't be able to use it anymore?
If you had "Find my iPhone" enabled on your stolen phone, you can log into icloud.com with your AppleID and lock/purge the phone from there.

If you haven't enabled that option or if the phone is not connected to the internet there is no way to access the phone and purge its content.