You'll find that, in the real world, it's not all that difficult, but I highly recommend that your read the pertinent information on Italian trains on the Seat61 website. Just scroll right past all the information that doesn't apply to you.
It appears that all trips from Venice to Varenna require a change in Milan. On some, you must also change in Bologna. You probably want to avoid trips requiring three transfers. I spot-checked some morning departures and found that the transfer times were very generous (40 minutes or more). The negative side of that is that it makes the trip longer. The positive side is that it gives you all the time in the world to figure out the train station and where you need to go to get the next train.
Larger stations (probably including Venice, Bologna and Milan, but I honestly don't remember) have a large central departure board ("Partenze") that will--at some point before your train is due to depart--tell you which track the train will be on. If the train is late there will be a notification about that, too. The large board will be in the ticket lobby, not out by the tracks.
I think there will also be a small electronic sign at each track, identifying the destination. I use that as confirmation that I am in the right place, but I don't usually wander around large stations looking at all those signs; I just wait for the information to show up on the Partenze board.
I don't remember whether there are announcements in English (possibly for major trains, especially), but it's not necessarily easy to hear them.
Although you certainly might find an English speaker at the information counter (which might be a labeled regular ticket window), it's usually not a problem to get the information you need by showing your ticket to a uniformed TrenItalia employee and asking "Do-VAY?" ("do" rhyming with "go"; it means "where?"). If that doesn't work, use "Kay bee NAH ree oh?" ("Which track?"). Understand that the track may not have been assigned yet if you inquire too long before the train is due to arrive.
If there are no employees around, look for a friendly young person who seems local; the younger folks are more likely to speak English.
If you have assigned seats (required on the fastest trains--which could theoretically sell out; not available on the regional trains--which therefore cannot sell out), try to board the right car at the beginning. Dragging suitcases through multiple carriages, looking for your assigned seat, isn't a lot of fun.
IMPORTANT: If your ticket (even for part of a journey) doesn't show a specific date and departure time, you must validate it before getting on the train. Look for a small box with a slot you can slide the ticket into. (Watch what locals are doing.) Riding with an unvalidated ticket is akin to riding with no ticket at all. I don't know what the fine is, but you do not want to find out.