How do I make a reservation

Online or walk up.

I believe you can pre-purchase a ticket online I believe. But you don't need to reserve a specific bus. So long as you have a ticket, you take the next available bus.

You can purchase your ticket at the station. You can get a round trip or one way. If you don't purchase a round trip and your planning on taking the bus back from the airport, you'll pay when you arrive at the station.