The best advice is to ask around when you are in Van. In Turkey, most bus companies have offices in the city centre and you can ask about times anc cost. In my experience, if they don't run buses to your destination, they have no problem telling you which other companies do. One advantage of booking through a city centre office is that usually they have free shuttle buses tot he out of town bus station.
Alternatively, make your own way tot he bus station, and there will be plenty of people to tell you how to get to Mardin, although you may have to wait a few hours.
I had a quick look online at the various bus ticket websites (www.neredennereye.com is useful, as is http://www.onlineall.net/otobusbileti/ which lists all the bus companies that allow online booking), and there doesn't seem to be a direct service between Van and Mardin, so you'll probably have to take a bus to either Diyarbakir or Batman, and change to a Mardin bus there.
Star Diyarbakir www.stardiyarbakir.com.tr has three buses a day from Van to Diyarbakir. Best Van and Vangolu probably do too, as well as Metro, a national company (see the booking sites above to find their websites)
I haven't been to Van for 13 years, but was in Mardin not that long ago...they're both beautiful places to visit, so hope you have a good trip.
Hi you dont need an official schedule ..You have to take the bus For Diyarbakır or Batman
Both of this places have acces to Mardin Batman-Midyat-Mardin or Diyarbakır-Mardin ..