Losing your phone or having someone steal it is a nightmare. Fortunately, Google has baked some tools into your Android that could help you find it. Or, at the very least, it'll help you wipe all your data and content so that whoever took it can't wreak further havoc.
Here's how to set it up:
First, navigate into your phone's settings. From there, find the "Security" section:
Next, look for a section called "Android Device Manager."
If you're not running the latest version of Android (which I'm not), you might have to first enter a section called "Device Administrators," or something like that:
Make sure that the box for "Android Device Manager" is checked:
Checking this box will let you remotely locate your phone via GPS, force it to ring for five minutes (particularly helpful if it's just lost in your room, or if you left it in the back of a cab and want to get the cabbie's attention), and allow you to remotely lock the phone or erase all your data: