The bus lines from Lisboa, Cacilhas and Setúbal are explored by two campanies, TST and Covas & Filhos.
The ticket price from Lisboa is about PTE: 570$00 (Euro: 3,-).

From Lisboa
The bus departure in Lisboa is from Praça de Espanha, just in front of the exit from the subway station with the same name.
From the Airport you can reach 'Praça de Espanha' by taking the Aero-bus (ticket costs PTE: 400$00 (Euro: 2,-), but is valid for all buses in town for one day), get out in 'Campo Pequeno', and then take bus line#56, and get out at 'Praça de Espanha'. A bit ahead of the bus stop where you get out you'll recognize the subway exit, and to the right of it the bus departures to Sesimbra.
Alternatively, if use the Aero-bus to go downtown, you can then take the subway blue line direction 'Pontinha' and get out at 'Praça de Espanha'.

From Cacilhas
If you want to take the bus in 'Cacilhas', which is already on the other side of the Tagus river, you can reach it by taking the Aero-bus from the Airport to 'Cais-do-Sodré' (last stop), or the subway from downtown to 'Cais-do-Sodré', and then take the boat to 'Cacilhas'. On the exit of the boat you can find the bus stop to Sesimbra.

You can also take a taxi, but we do not advise it, since it will be very expensive (about PTE: 10.000$00, Euro: 50.-), and much dependable on the traffic and way chosen. The driver can decide to take a road with much less traffic (through the new bridge 'Vasco da Gama'), but that is twice as long, than the normal way (through the old bridge).
If you choose to take the taxi in the Airport, please make sure that you use a standard taxi (black+green roof or all-beige, marked 'taxi' in the doors and 'taxi' light on the roof), that the driver has started the meter when you get inside, and refuse to pay whatever is not in the meter (the rate in the town will change close to or on the bridge to the 'rate for outside Lisboa'). Luggage in the trunk can increase the price by PTE: 300$00, and the return fee is already taken into account in the meter rate.