You need to activate Find My iPhone (in Settings>icloud), and if the device's batteries still have a charge, the device is not turned off and has not been reset and is connected to wifi, then you can try to track it.
Using a computer, log into icloud.com and look at the Find My iPhone page. On a device, invoke the Find My iPhone app (get it for free at the itunes app store) and log into the icloud account. There, you can track your iphone and do various things like put it in "Lost Mode".
If you don't use a passcode that must be entered each time the device is turned on (you get used to this), that's like leaving your house/appartment unlocked whenever you leave on a trip.