One of your first stops should be Center for Maine History. Here, you'll be able to orient yourself and get some great tips before you set out exploring.
This Downtown Portland tourist information center is an ideal place to kick off your day of sightseeing. There's loads to explore in the vicinity, such as Portland Head Light, Portland Museum of Art, and University of Southern Maine. Wander around at your leisure at Center for Maine History before hitting up these other fine sights.