If you really want to stimulate a G spot during sex, you need to aim for a sex position that is going to put the girl on top. This is the best way to get the penis to the G spot because the woman gets to control the action. She can move her body the way it needs to move so that she gets sexual pleasure, and she can basically use the penis like a sex toy. There are more girl on top positions out there than you think, and some of them may be fun to play around with. Here are just a few of them you may want to try.
The classic girl on top sex position is the cowgirl. This goes by many other names, like stallion or rider, but it is the same regardless of what you want to call it. In this case, the man lies on the bed with his erect penis in the air, and then the woman straddles him with her vagina just over his penis. With her knees on the bed, the woman lowers her body onto the penis and then moves so that she can get pleasure from the G spot. This is position that every person should try at least once in his or her sex life.
Another option for having the woman on top for sex is to actually have sex at the edge of the bed. The man can sit up with his legs dangling from the side, and then the woman can straddle him with her rear on his knees. Most women at this point will almost ride a man like a bull, thrusting their pelvises back and forth, rather than up and down. The movements that a woman will make will depend on what fees good for her G spot.
You can also look into a variation of cowgirl that may involve a little more physical activity. The Asian cowgirl, for instance, is basically a squat over the penis, where the woman has her feet flat on the bed instead of her knees. The knees are bent, and then she can pop up and down on the penis. There are tons of other options for G spot stimulation, so you just have to play around to see what will work for you. There is always a new sexual activity to try.