There are no buses or roads that go to Aguas Calientes, the stopping-off point to access Machu Picchu – at least not all the way. The closest you can get via bus or taxi is Ollantaytambo, and from there, like everyone else you have to board a train.  For those wanting to board a train directly from Cusco, there are two options.
At the San Pedro Station in Cusco, at the intersection of Cascapara and Santa Clara, PeruRail has two daily departures, a “Vistadome” which leaves at 6:00 am and arrives at Aguas Calientes three-and-a-half hours later, and a “Backpacker” which leaves at 6:15 am and arrives in four hours.  The Vistadome at $110 is the luxury trip, includes snacks, beverages, and a sky dome (ergo the name). 
The Backpacker, at $75 is the budget trip, but still comfortable.  The return trips leave at 3:30 pm and 3:55 pm respectively. There are also morning departures from Aguas Calientes, beginning at 5:45 am. Schedules are subject to change, however, so one is best advised to purchase ahead. And be prepared to bring cash – credits cards of any sort are not accepted.
Those who can afford it and want four star luxury and convenience can take the new Hiram Bingham Deluxe Train, which departs from the city of Poroy, about 20 minutes north of Cusco. This train leaves at 9:00 am and gets to Aguas Calientes at 12:30 pm. The train, complete with a bar car, is modeled on the classic Orient Express train of its eponymous owner. The $470 price includes exclusive buses to the ruins of Machu Picchu and guided tours.  The trip back includes cocktails and a four-course supper.
Once you are in Aguas Calientes you must head over to the Tourist Information office at their Plaza de Armas, and purchase two things: 1) a three-day access pass to the ruins, priced at $43, and 2) a round-trip bus pass for $12. The bus itself leaves from the Plaza.