if you have icloud set up you can use www.icloud.com and try and track it, and lock it.
If you can't track it then you have to wait until it connects so you can attempt to.
You can report it lost/stolen to the police
T-Mobile will disconnect service from the sim card to ensure no unusual purchases or usage happens if you call in and tell them. They'll also block the IMEI so it can't connect to T-Mobile's network.
Manage your T-Mobile account