One of the most important prerequisites is that your Galaxy is A. connected to your Google account and the stolen phone is not powered off; B. pre-installed with some tracking apps and the stolen phone is not powered off.
If condition A is satisfied, you can use Google Device Manager, which lets you ring, wipe off or detect the last position of your phone with another device. It's free and you don't need to install anything on the lost phone.
If condition B is satisfied, hmmm, I think you already knew what you need to do...