You need to get the Light Rail Metro from Porto Airport to Porto - Campanha Station via Line E.
See:
http://www.metrodoporto.pt/pagegen.asp?SYS_PAGE_ID=871913&id=154
and
http://www.metrodoporto.pt/document/827457/874411.pdf
You then have the choice of rail and two bus services to Lisbon. In my opinion rail is faster.
Rail:
You can find a link to the High Speed Porto to Lisbon rail timetable on my blog here:
http://barlaventostravelblog.blogspot.com/
Scroll down on the right hand side panel till you come to:
»» Algarve & Portugal - Travel Transport Links
»» Continue scrolling down in this section till you come to:
Porto - Lisbon - Porto Train Timetable
If you are travelling in high summer you should book in advance if possible but the Portuguese railway online booking system is not that great.
You can try it here at www.cp.pt.
The price at present for the service in question will be Euro 27.50 but this may go up soon.
I have also published here on my blog the current full Porto Campanha to Lisbon Entrecampos (Change at Gare Oriente:) here:
http://barlaventostravelblog.blogspot.com/2009/08/porto-campanha-to-lisbon-entrecampos.html
I do not recommend the bus as the bus stations are nor easily accessible with baggage and if you are in a hurry the train is best.
