Trains

All depends on which train you buy tickets for.....make sure you are putting in the city/station names in Italian into the fields at:
https://www.trenitalia.com/en.html

There are different types of trains, usually the fancier ones go faster and are more expensive.....I haven't looked for tickets, but I am assuming the ones to various towns on the lake would be serviced by slower trains, regionale trains....

Yes, you will have to transfer in Milan. You switch from an inter-city train to what is basically a commuter train

Hello mrenberg169, and welcome to the boards!

To travel to Lake Como from Venice you would take a high speed train from Venice to Milan and then a local train to your LC destination. Are you actually interested in going to Como the city itself or a smaller, vacation place such as Varenna?

The state run Trenitalia https://www.trenitalia.com/en.html is the best source for buying tickets. You would go to their website and enter Venezia and your destination like Varenna whose station name is Varenna-Esino and the date you want to go.

There are third party ticket resellers and services such as Rome2Rio.com that can make searching easier, but eventually buying the actual tickets are cheapest direct from the source.

There are fast trains for the Venice to Milan section. There are two fast train companies in Italy - the state run Trenitalia https://www.trenitalia.com/en.html and the private .italo https://www.italotreno.it/en

The earlier you buy you fast tickets the cheaper they will be so avoid buying "day of" if you can. Because they have assigned seats it is possible for these trains to sell out but that would probably only be on the busiest days of the year. Check the details of the different tickets - the cheaper the tickets the more restrictions about changing them later so you're trading price for flexibility.
It's really pretty straightforward just check the schedules.

Local, slower regionale trains run all the time and you can buy those tickets whenever you need them.

Hope that helps - have a great trip,
=Tod

The regional trains serving Como destinations, such a Varenna, are often crowded and do not sell reserved seats. We usually have found seats on the train TO Varenna from Milan Central, but have found the hour train FROM Varenna to Milan Central to be standing room only. Just to prepare yourself. As previously stated, these are commuter trains.
Have a great trip!
Safe travels.