first congratulations for deciding to take a cruise, second welcome here to the cruise-chat. In this chat you will find all answers to your questions.
So: You want to stay at Venice for some days. Than got to "Port Hotels an Transports" in this chat, click on the icon "FIND", write "Venice" and submit. You will have a lot of answers. If you find no answer for yourself, please ask me. So I can give you some hints for a cheap hotel and so on.
The Ports. Do it same was. Go to "Europe port chat" and look for your believed port. Tom Ogg has a list of ports with excellent advices.
Venice you can visit by your own. Bari: stay on the ship. Katakolon: take a taxi to Olympia. Istanbul: take a guided tour. Dubrovnic: Your ship will tender you to the ancient port. From there you can go around the marvellous city. There are a lot of hints here in the forum. Tale the tool "FIND".
Have a nice first cruise. I did this route with my family in 2004...