money. phone usage, storing bags and trains

My husband and I are taking our first trip abroad to Italy in August. We have a few questions.
First - We will exchange our currency for euros when we arrive in Italy, however should we has some euros with us when we leave the states and how much should we excahnge at one time?
Second - we want to be off the grid when we are vacationing but know we will need to have emergency contact. Can we easily make calls on our cell phone or is it easier to just text?
Third - We arrive in Venice but our hotel is in Florence. We want to spend some of the day in Venice. I have read that we can store our bags at the train station. How do we do this? Are our bags safe.
Four - traveling from Venice on our first day in Italy - does anyone know how long the ride is from Venice to Florence? Can we catch a train in the early evening? What kind of transportation should we get the train station to our hotel in Venice?
As you can see we are true novices. We have to start somewhere!!! Thanks for any help you can give us. Anne