When a ride pops up on your screen and you wish to accept it, press the 'Accept' button. When you get to the pick-up point, press the 'Arrived' button at the bottom of the screen. This will send the passenger a notification that you have arrived and are waiting for them.
If however, you wish to reject a ride, press the 'X' at the top right of the offer card and the ride will be removed from your screen. Please remember to set yourself to 'Busy' if you're unavailable to take any trips.