Hi, and welcome to the forum.

As an American, you'll have to go through the visa process to be allowed to stay in Portugal for the "long term" (usually anything more than 3 to 6 months). You should contact the Portuguese consulate in the US for information about visas. You may also want to look at the EU website EUROPA - Gateway to the European Union where they have files on living and working in the individual countries in the EU.
Cheers,
Bev