You get to Aguas Calientes by train. It’s best you reserve in advance (through email and continue to email back asking if youre still reserved) and than pay for it and get it once you get to Cuzco (get it immediately after you arrive in Cuzco, that is what I did).

Once you arrive in Aguas, you will go through the little markets to get to the plaza. To get your Machu Picchu entrance tickets, there is an office like 3 or 4 doors down to the right of the only church in the plaza. On Oct 2007, it cost me about $41.

To get your bus ticket to Machu Picchu, go back down towards the train tracks, the bus ticket office (blue building) is along those tracks. It costs $12 roundtrip.

We had a wonderful time in Aguas Calientes and Machu Picchu but encountered a few difficulties that I'd like to warn other travelers about. In all my trip preparation including extensive use of Trip Adviser, these items were not covered.
1) Entrance: Only 3,000 visitors are admitted to Machu Picchu (MP in future) daily. If you're not with a tour group, you will need to buy your tickets 1-2 days in advance at the town Cultural Center, near the square in Aguas C. We went about 6 PM and could not get 5 tickets for the next day so had to wait till 2 days. There may be some way to purchase in advance, but I did not encounter it.
2)Train: I purchased my Perurail tickets via website and wired money 4 wks in advance. When we went to pick up our tickets in Cusco on the morning of our train travel, they were sold to others. We were told that tickets must be picked up one day ahead (although this was never mentioned on the website or in any communication). Plan to have a local person pick them up for you- like, ask your hotel- or plan to be in Cusco one day in advance. The ruins around Cusco are interesting, so you can easily have an enjoyable day there. Plan at least one overnight at Aguas C. so you can spend a full day at MP.
3) Hotels in Aguas C. We stayed in Apu Majestic and El Parque which were adequate but each had problems. Service and staff excellent at both. The winter is very damp. At El Parque, the room with a dehumidifier was OK and the room without was moldy. Watch out because the hot water is very limited. At Apu Majestic, most of the 2nd floor rooms have no windows and the first floor rear room has a real ugly view of a neighboring ramshackle building. Ask for a corner room on the third floor and you will be safe. Breakfast was very nice at Apu.
4) If the weather is foggy/cloudy/ you will not see a sunrise at MP, but it's still worth going up very early so you can be one of the 400 people who get to climb Huanapicchu daily.
See my review of Huanapicchu (also spelled Wyana picchu).