Plenty of trains to Como from Bergamo. Very easy and not expensive.
I would highly recommend to stay in Bergamo for a night as well, lovely town and restuarants/bars in the upper side. Narrow cobblestone streets etc.
Have stayed in the Accor Hotel just 2 mins walk from the trains station a few times. Very very nice, modern and reasonable especially if getting in on late Ryanair flight.
Can walk down the street and then catch funicular to the upper town. Many peeps seem to fly to Bergamo and head straight to Milan but miss Bergamo.
Bellagio is very very nice but I found it boring, spent the whole time going up and down the lake on the ferries and stopping off in towns for a beer or coffee. Thats the type of holiday it is, lazying around. Ok for 2/3 nights. Boats run until 11.30 or 12 or maybe 12.30 so it was good fun. A one or day ticket was very reasonable. Can base yourself in Bellagio and go to another town for the night and get ferry home. That was cool.
Very beautiful, bring your swim togs for the lake.