I'd be inclined to do an internet search for pictures of impact craters and go from there. Style-wise, I can't give suggestions unless I see what the map it's being placed onto. But an image search should give you some ideas... notice the way the light and shadows play across the crater. That's what will define the shape. Then just apply whatever kind of line work and shading your map has to depict it so it fits the style. And, welcome to the Guild. =)