Ultimately try different stances and find out what feels best. If you're looking for where to start, I can give you this advice. When you're just standing up straight in your living room, a narrower stance will feel more comfortable. When you're actually out riding with your knees bent and needing to balance on something other than a solid floor, then a wider stance will feel more comfortable than whatever stance felt best standing up still in your living room.
If your board is the right size for you, then you will probably end up riding the middle or outer holes...most likely not the inner ones unless the board is too big for you. As for angle, most wakeboarders ride symmetrical duck stance. I see quite a few other riders that like big angles like 18 degrees, but I prefer something like 12 degrees.