The best is to get the train to Pompeii Scavi, walk down to the square and catch the bus which stops immediately outside the Pompeii tourist office situated on the "island" in the middle of the square. Despite what is written on the Vesuviana Mobilita leaflet an E3 bus ticket for 1.80Euros (see tourist office leaflet on ticketing) is good for one way. For the return you can buy them anywhere before you go up but I suggest you get them from Andrea de Gregorio's souvenir shop (see my things to do "active volcano" tip)
Vesuviana Mobilita Info-point: 081 963 44 20

