How can I ping my lost phone or get the last GPS location?

Go to http://www.icloud.com, log in with your Apple ID, and select Find My iPhone. Find my iPhone will only work if all of the following are met.

1. Find My iPhone was setup prior to this loss
2. The device is powered on
3. The device has an active internet connection, either cellular or wi-fi

If any of those are not present, then you will not be able to track the device.

First try the steps that ChrisJ pointed out.

If the device isnt found or doesn't have an active internet connection, you can initiate lost mode and it will notify you the next time it comes online active: https://support.apple.com/kb/ph2700

Then the only other choice to locate it is to physically look for it. If you check Find My iPhone, it should have shown the last location.