Artlover,
We made the same trip a few months ago. The line at the change desk for the bus ticket will be pretty obvious, and the bus just outside (Treviso airport is tiny). The bus drops you right at Pl Roma (basically a big parking lot at the end of the Venice causeway). The ACTV office is on one side, and the vaporetto stop is just past. If you arrive in the evening, be aware that some of the vaporetto routes change at night (look for the N on the boat number sign)- the system is a bit obscure to the visitor; just know the stop you want and ask which boat number when you buy your pass. If you go the wrong way, just count it part of the trip.

The easiest option is the water taxi. It should cost about 80 euro for up to six people and their luggage. Don't know if recent increases in oil prices have changed this.