The question you need is:
Dove vendono i biglietti di corriera/autobus? (Where do they sell bus tickets?)
The buses that travel up and down the lake are more like coaches than city buses - hence they are usually referred to as corriera and the city buses in Como are called autobus.
The tickets are sold according to distance. Tariffa 1 is the shortest distance, probably two or three villages. This ticket would take you from Tremezzo to Menaggio (a distance of about 5 km). Tariffa 5 would take you to Como. Tickets aren't dated, so it is best to buy a good supply:
Dieci biglietti tariffa uno, per favore. (10 tickets tariff one, please)
When you get on the bus, the driver tears off the bottom tab and you keep the rest. The tickets are valid for one journey so if you are going there and back, you need two tickets. If you haven't got a ticket, the bus will stop at the next place that sells them and you will have to hop off and buy them (holding up the bus). In cities, it is a different type of ticket and you have to validate it in the punch machine on board.
Now you can see why it is a pain!