How to set bulb mode on nikon d5100?

How to set bulb mode on nikon d5100 on the way to press shooting button to start capturing and press again to stop it. I need longer exposition and I can't hold shooting button for a 100 minutes. So I asking is there option to set bulb mode, press button for capturing and press again to stop it?

100 minutes why?

You must be trying some pretty far out 'experimental' shot.

I honestly don't know if that length of exposure would be good for the camera, could you come back and tell me, I've got one too.

You need to use a remote cable or ir remote to do what you are attempting to do.