when in Venice its best to use your feet or try a water bus ,remember there are no roads in the centre its all canals and little passages leading to squares. to travel by gondola is very expensive thats why you will always see them with about 5 people in them.also when you visit st marks square check the price at any of the cafes as they are also very expensive,it gets cheaper the further away you go from st marks square.other than that it is a beautiful place to visit.don't forget to take a camera ?
it's quite compact and walkable; but get a good detailed map. Water buses will get you along the main canal quickly. Visit St Mark's square and the buildings round it - the cathedral and doges' palace especially.
ps .at st marks square you can vist ,the clock tower,st marks cathedral,the bridge of sighs,the doge's palace the rialto bridge is nearby.there are lots of nice little squares not far away where you can get a drink of tea and a bite to eat that are cheaper than st marks square.check out your local library to see if you can get a travel guide..