To begin using Android Device Manager, go to https://www.google.com/android/devicemanager and log in with your Google account. Note that you may get a prompt asking you for permission to allow Android Device Manager to use location data. Click on the "Accept" button to continue. It's that simple. However, if you want to be able to remotely wipe your device from Android Device Manager, you will need to activate that function on your device.
On the Google Nexus 7, you will need to go into the Google Settings app on your device and then click Android Device Manager. Once you check off the check box for Allow remote factory reset, you will be able to wipe your device preventing anyone from getting to your data.
On the Samsung Galaxy S4, then you need to go into Settings and choose Security. Then select Device Administrators. When you touch the box to check off Android Device Manager, you will be taken to another screen that you will need to activate the service. Once that has been done, you will now be able to remotely wipe your device.
Needless to say, Android Device Manager is a simple way to track and remotely erase your Android devices if it is lost or stolen. I can see this service adding new features and getting better with time.