hey, im also traveling to portugal soon (im portuguese)
anyways, from personal experience, when you get out of customs at the airport, right at the arrivals hall look for convenience stores/kiosks that sell sim cards with data plans, you may have to present your passport though. make sure you do this at the airport coz you are more than likely (next to 100%) to able to talk to an english-speaking agent/seller at the airport than outside