Make sure your location is activated in your phones settings. The phone needs to know where you are. Start the app and navigate to the section where you can define a new field, or edit an existing field. Enter the map and zoom in until you find your field, or a location close to your field. Pinpoint the location by pressing for two seconds. You will get two coordinates which represent the latitude and longitude of the selected location. This are the GPS coordinates that you selected. You can also tab the "use this location button" if you are present at the field that you are entering.