From Interlaken Ost station, there is an hourly train to Lauterbrunnen (make sure that you are in the right part of the train, as the train divides at Zweilutschinen). At Lauterbrunnen, follow the directions. You will make the ascent to Grutschalp in a cable car. From Grustchalp, a train takes you along the ridge to Mürren. On a clear day, the view from the train is spectacular.
Mürren is a beautiful car free resort, every thing is accessible on foot. In the winter season it is a very popular ski resort, with plenty of rental shops. It also has a few sledge runs which are great fun.