Take the vaporetto or you can walk if you travel light.
The vaporetto #1 goes to San Marco Vallaresso terminal and you cross the Grand Canal with spectacular view.
If you want to walk, it would take you half an hour to reach San Marco Square or even more. Reach and cross the bridge of Santa Lucia train station Ponte degli Scalzi (bridge of the barefoot), then walk until you find Ponte delle Guglie. After you have crossed this bridge turn right and walk along Lista di Spagna which will lead you to Campo San Geremia and Ponte delle Guglie. Then take Strada Nuova to Rialto and then to Piazza San Marco.
Or from Piazzale Roma follow the signs to RIALTO – SAN MARCO. It is very easy. You will reach Accademia bridge and go across to Campo San Stefano. From there you will reach San Marco Square in a few minutes. Always follow the signs.