What is the easiest way to get to mantova from Milan? and How many days would you spend there?
Trenitalia shows 1:50 minutes from Milano Centrale. This is on a regionale train and costs about 11 euro. There is a faster train for part of the trip but it requires a change of trains, and the total trip is longer, so I would go with the direct regionale.
I love Mantova and I would stay two nights, more if you wan to visit some of the other beautiful cities nearby. Verona is a little over an hour away by train, and Peschiera del Garda is a little beyond that. Sabbioneta is also a lovely small town to visit.
If I were you I'd visit Cremona as well. Close to Mantova, en route from Milan.