Find my iPod Touch (iOS 4.2.1 and greater) - http://images.apple.com/ipodtouch/built-in-apps/find-my-ipodtouch.html - has to be enabled before device is lost. It also requires the device be on and connected to registered WiFi network, and not having been restored by the finder (if it was really stolen then the thief has probably restored it). If you have activated the "Find My" feature, go to https://www.icloud.com or use the Find My iPhone app - http://itunes.apple.com/us/app/find-my-iphone/id376101648?mt=8
Setting up iCloud (requires iOS5) - http://www.apple.com/icloud/setup/ios.html - includes activating the 'Find My' feature.
Frankly in the case of a stolen iPod this almost never works. Report it to the police. Change passwords on accounts. Go shopping for a new iPod.
Kappy's missing idevice recommendations 09/2012 - https://discussions.apple.com/message/19569712