You don't, you run an app that uses it. If you just want to know where you are, Maps will show you a map with your current position indicated. If you want to see your latitude and longitude, GPS Status (free in Play) will show you that and more.
If you're asking how to enable GPS, you can put it on the Notification pulldown, or go to Settings/Connections/Location services/Use GPS satellites. (Turn that on and the GPS receiver is on. You won't see anything on the screen, though, unless you're running an app that's using it.)