Although many people who move to Portugal live comfortably without speaking a word of the language, it’s still a good idea—and also just polite—to learn at least some basic Portuguese. Fortunately there a lot of tools to do this, as well as many Portuguese who moonlight as language teachers or seek out tandems (language exchange pairs) as a way of improving their own English.
For anyone who wants to get a head start before moving to Portugal, Practice Portuguese is an excellent resource for online courses. Many people also experiment with Duolingo, but that unfortunately teaches Brazilian Portuguese (although even the first level contains many entertaining phrases, some of which one hopes never to hear, such as “there is a knife in my pants,” and some that in fact seem pretty okay, like, “she is a good journalist”).
If the Duolingo style of language-learning works for you, there’s a similar app called Memrise, which is oriented around Portugal’s Portuguese.
For in-person lessons, some of the better language schools include Lusa School, in Lisbon, which offers group and private lessons; Faust in Quarteira (Algarve), which is recognized by the Ministry of Education in Portugal; and Portuguese Connection in Lisbon, which offers extensive and immersion courses. All of these also have online classes.
The Portuguese government, in partnership with the High Commission for Migration, also offers free courses at schools around the country. Most junta freguesias (neighborhood or regional government) also offer Portuguese classes for foreigners living in that district.
The hardest part of learning Portuguese is practicing outside of class, given the prevalence of English in Portugal. Facebook and Meetup promote language exchange events, and tandem.net is a good resource for finding partners for one-on-one language exchange. And at least one single woman who moved to Portugal found some luck with dating apps as well, arranging to meet for coffee and practice (with clear expectations, of course) whenever she matched with someone who didn’t speak much English.