Hello Manoi,
There are numerous trains that leave Milan for Venice, but I recommend taking the earliest direct train since Venice gets very crowded. Beat the tourist rush by getting there before the tour buses do.
If you have time I really suggest getting to Venice early and then staying at least one night. After the tour busses leave, Venice is left to Venetians and you can walk the narrow streets and alleys without the masses of tourists. Venice does not really have a nightlife and everything closes around 11 PM but a walk along the canals at night will give a new meaning to the term "La Serrenissima"
The Italian Train network has a website where you can check train timetables in english:
http://www.trenitalia.com/en/index.html
Good luck, have fun and take lots of pictures
justin