Just play with it I think most people start a bit more of a front side stance. Would suggest starting at 15/0 and play with it from there. Just get your bindings on the board. Do not tighten down all the way yet just get them snug. Strap in and alternate from toe and heel side edges. Can do this on a rug at your house. Would suggest to try the 15/-15 try 15/-6 and 15/0 and see which one feels more comfortable for you.
Once you get a range that makes you feel comfortable tweak from that. So if you feel good at say 15/-6 take it to the mountain that way (with bindings tightened up all the way). Then play a bit more with it and try 17/-6 or 15/-9 might take a few tries to get it right for you.
a very standard startup is 15,-9. this is the typical start off point that most people who know what they are doing reccomend. after 15,-9 you can really feel as to where you want to go but it does put your feet in a nice and very comfortable position.