The are several options for you, depending on the use that you wish to give to it.
You have Sapo, Clix, Novis or even PT as ADSL providers or Netcabo and Netvisao as cable service providers.
For ADSL you will always need a phone line supplied by PT-TELECOM.
I use a Netvisao service because of the package of TV + Internet.
I came to find that between all of them the price doesn't differ much its just a question of comparing the type of services offered.