first of all sorry you, that's one of the worst things that can happen to a phone.
if you think the theft is using your phone only with a different simcard you have the following options:
- try planb
(as already mentioned above) to locate your phone.
- if you have a google account on the phone, try android lost
that will gain you the full control over the phone including locating it, taking photos of the cameras and wiping the complete data of the device out. its a really great tool .
- if you have a Samsung
account on your phone, try the samsung dive website
with this you can also do things like locating or wiping data view call logs and so on.
if all the above fail you should change the passwords of all your accounts used on the phone, especially the email accounts. also try to contact your provider, really few of them provide a location service based on the imei number of the device. and also some of them offer the service to put the device's imei number to a ban list so the phone is useless at all because it cant login in to the gsm network.
*EDIT*: go to the police and let them give you a document that the phone is stolen, and the imei and serial numbers. give this document to samsung and they will put the device to a blacklist, so the device wont get service/support/repair anymore.
i hope some of this become useful to you