A good position is you on top, her lying flat on the bed, legs in the air and spread apart. Go deep, hold still, let her grind her hips and feel her shaking in intense pleasure under you.
I've read you can even reach it with your fingers, but if you enter her then the vagina tents and becomes deeper. I think 8 inches should be fine with most girls, but even 6-7 could do the job. I think more girth might make it harder to maneuver inside her.