To begin with, could you clarify if you're arriving at MXP on a flight from the U.S. or on a budget airline? You may be arriving at either Terminal 1 or 2, which will affect the method of transportation from the airport.
If you're arriving at Terminal 1, the easiest method of transport to Milano Centrale is the Malpensa Express. Note that they have two routes, one going to Centrale and one to Cadorna station, so be sure to board the correct train. I'd suggest buying all your tickets at the airport station when you arrive, for the reasons you mentioned either from a Kiosk or a staffed ticket office. If you pre-purchase tickets for the premium trains and you miss your assigned train because your flight is late, the tickets will be worthless and you'll have to buy more.
A few other important points regarding the trains. Prior to boarding the Malpensa Express, you MUST validate your tickets or you'll face hefty fines which will be collected on the spot. For the journey from Milan to Venice, reservations are compulsory and your tickets will be specific to a particular train, date and departure time (reservations will be included with the ticket you buy at the airport and you'll be assigned a car no. (Carrozza) and seat no's (Posti). You can NOT use any train other than the one specified on your ticket, or again hefty fines! Note that your destination station will be Venezia Santa Lucia.