The closest major airport to Truro is Logan International in Boston. From there, you can rent a car and drive to Truro (this takes about two hours). Alternately, you can take a high-speed ferry or fly to Provincetown and rent a car there. You will most likely need a car once you are out on the Cape in order to get around.
Provincetown Ferry: Bay State Cruise Company or Boston Harbor Cruises
You will need a car out on the Cape, so here are lists for Logan Airport in Boston and Provincetown:
Logan Airport car rental list
Here is a taxi company for anyone who doesn't feel like driving (or doesn't want to designate a driver) to use: Cape Cab (508-487-2222)
Here is a taxi company for anyone who doesn't feel like driving (or doesn't want to designate a driver) to use: Cape Cab (508-487-2222)