TST buses take you to Lisbon and to neighbouring places such as Almada or Cacilhas. It is cheaper if you buy tickets in advance (e.g., from the kiosk on Praça da Liberdade nearest the main road, or from Papelaria Hlucha, near the bus stop for Lisbon).
The bus will take you to Praça de Espanha, Lisbon, in 25 minutes. You will need a ticket with 4 "modules", one way.
If you don't want to catch a taxi to go to the airport from Costa, the best way is to take the bus to Praça do Areeiro and to take a taxi from there (around 5 euros). If, however, you travel during the rush hour, take the train from nearby Pragal station.
From South bank of the Tagus river you can get to the center of Lisbon by ferry for just 0.74 EuroCents for one way.