Q: my ipod touch 4g got stolen and i dont know how to activate find my ipod app

Your "Find my iPhone" apps has to be set up and activated prior to the loss, if you did that, you can log into iCloud from your computer, start "Find my iPhone" and your device will be shown on the map, if it is connected to a network. More info here:

iCloud: Troubleshooting Find My iPhone, iPad, iPod touch, or Mac

Apple - iCloud - Find your missing Apple device.