In a front of Stazione Marittima is an information kiosk. People are very helpful there.
From Cagliari there are not too much opportunities for a day trip by train. The timetables offen don't match with the ones from the cruise lines.
From the ARST bus terminal (near the train station and not far from the harbor) you can take a bus to Nora.
PF and PQ buses run very frequently between Cagliari and Poetto beach.