Public transport for Venice with the exception of trains terminating at Venezia Santa Lucia and buses at the Piazzale Roma is by waterbus - vaporetto - or water taxi. You could look for Vaporetto maps for Venice.

As you don't say how long you will be in Venice, there's no way of helping you with your second question.

Finally, there ISN'T a big supermarket next to the railway station; Venice being an island, all 'shops' are little ones with their goods being transported by water hence pricey. You'll find bigger supermarkets in the town of Mestre which is across the causeway on the mainland opposite Venice. Billa and Coop should suit your immediate needs unless you plan to shop for a month's travelling at once. Where they are will be shown on a Google map of Venice ... not necessarily next to the train station.