The Gull bus service to go the airport is a nice way to travel. The seats are comfortable and from Geelong they will only make a further two stops between there and the airport. They will drop you right out front of either the domestic or international terminal (the same places for departures from the airport as well).
From the Gull depot it takes 1 hour and 20mins to get to the airport. In Geelong you can get on at McKillop St or arrange to be picked up at the Geelong train station.
Most tickets you can buy on the day, but if you are departing from Geelong on a bus that leaves before 8am, you have to buy the ticket at least the day before.
A one-way adult ticket is $30 (a little expensive, but not too bad).
If you are coming straight from Tullamarine airport in Melbourne get on A Gull bus which takes you to Mc Killop st. From there it is a short walk to the city centre and transport in every direction.
If you want to take in the city centre and waterfront go on foot. Its not far to anything and that way you wont miss a thing.