Place your feet at shoulder width or wider. You don't want to be doing the splits, but you want a good support base. Touching your legs together isn't going to provide that. Just do some progressively heavier warmup sets. You will get the feel for a comfortable stance.
Your grip should be the same as if you were doing them seated. I usually put my pinkies on the rings. Sometimes I will go a tad wider and put my middle fingers on the rings. This largely depends on how broad your shoulders are, but I would just go with shoulder width or a little wider; whatever is comfortable for you.
I'm not sure how much weight you should use. Just start with a warmup with the bar, and add 20-30 pounds at a time. Bang out like 2-3 repetitions at each weight. You will know when you're in your target rep range zone most likely.