van not starting?

Hi
Thanks for your reply. I charged the battery full, voltmeter said 13.6, put it on van, reading said 13.2, tried to start van, it turned over slowly for a couple of times then just clicks, took another voltmeter reading and said 12.4,
The battery and oil light is on now in cab, is that the battery or starter motor? any advice welcome
Thanks

dj
April 2012