From the entrance to Takao-san, the chair lift(or cable car) will take you a lot of the way to the top, although not all the way.
It's quite a long journey, and the scenery is nice. Once you have reached the top, it's about 40 minutes on foot to the summit.
900 yen return.
The entrance to Takao-san is easily reached on the Keio line.
We used the JR Chuo line as far as Takao, and then transferred to the Keio line which goes one stop to Takao-san guchi, from where you can walk all the way up the mountain, or get a cable car or lift part of the way up.