It is a new country for me and it is full of new places to travel. Luckily we have one friend at the apartment who knows the area pretty well. So we did not have much trouble in locating the places to visit but if you travel by public transport following information would be useful for you.
If you are near IST you need to travel from Alameda Metro station to Cais do Sodré station which is at the end of Green line. Then you need to get the Train from Cais do Sodré station to Cascais. To travel up to Cascais you can obtain a travel card from Metro station which covers all 4 forms of transportation including Bus, Metro, Train and Ferry. You have different options but it is better to buy a viva via gem card if you are a non frequent traveller. It is reusable and reloadable card.
Loading this card with 10 Euros would be enough as we experienced and you can top up it from train stations or metro stations conviniently. It is advisable to keep 5,10 Euro notes as sometimes it will issue receipt for balance which you should claim at train stations if you try to recharge from a train station. Believe me it is a hassle :(.
You should spend some time at Cascais as it is a beautiful area with lots of tourist and local activities. We stayed there before moving to Cabo da Roca.
This was truly a beautiful beach...
From Cascais you need to get 403 bus operated by scottrub. It is usually has one hour frequency. It will go via Cabo da Roca to Sentra and you have to get down from the Cabo da Roca on the way. These buses does not have display boards for the stops but Cabo da Roca is a special stop and you know once you arrive there and just enjoy the breathtaking views out side on the way from the bus. (Believe me it is a special stop ;) )
Cabo da Roca is the western most point of the Europe mainland. You will be surely amazed how picturesque the view is. You will lost in your thoughts for a moment when you reach the top. But make sure not to fall out from the top to Pacific ocean while taking selfies.. Cos' it is a shame that you cannot tell how magnificent it was to you friends :P