Your inquiry lacks specificity.
First, you'll need a Google Maps API key to display a map on your interface.
You can activate current locations by
GoogleMap.setMyLocationEnabled(true);
Once you've asked the user for their destination, you will utilize the Google Maps Directions API with a Server key to select a route in JSON or XML format via a URL.
https://maps.googleapis.com/maps/api/directions/output?origin="origin_location&destination="final_destination"&mode="traveling_mode"&alternatives=true&key="ServerAPIkey"
You should parse this URL for a string containing JSON or XML data.
Then use that data to create a polyline of your chosen route.
For more comprehensive information, it is recommended that you consult Google's developer documentation on their official website.
Cheers.