To get to the Kehlsteinhaus by bus (you can only hike it when all of the snow at the peak is gone, which happens in mid- to late summer, if ever), take a city bus (ask someone which one) from the
Bahnhof up the mountain. It will drop you off, and you will have to board a special mountain bus to make it the rest of the way. I bought the mountain bus ticket at the Bahnhof before I got on the first bus, but you may be able to buy them once you get halfway up the mountain. The city bus ticket you buy at the
Bahnhof, or you ride free if you have a day pass (a super option if you are coming from Salzburg).
Once you get to the Kehlsteinhaus, you have to book your bus back down (but don't worry if you miss it--you can just catch a later one if you decide to stay longer, unless your bus is one of the last ones). Then you take an elevator up to Hitler's house and are free to explore the mountain peak and take lots of pics. There is an outside restaurant serving typical German fare, which would be a great option for people wanting to enjoy the weather and lovely view for a little longer. I admired nature and the lovely mountain atmosphere for about 3 hours before I was ready to leave.
Of course, if the weather is nice and there is no snow, you can hike up the mountain from Berchtesgaden below. Get a good topographical map from the tourist office before and be prepared for several hours of hiking!
If you are interested in the history of Hitler's rulings from Berchtesgaden and the Eagle's Nest, there is a nice museum around the corner from where you board the mountain buses. There is a small admission price, but it is free for students.