I suggest starting with your fingers as they're much easier to manipulate than other parts of your body. Start by inserting your middle finger with your palm face up and feel around the top wall of her vagina until you locate her pubic bone.  Basically, the G Spot is located directly behind the back edge of her pubic bone.  Massage her there with the pads of your fingers (make sure you don't use your nails or you'll scratch her. Also, keep them trimmed short) by lifting up and pulling back against her pubic bone.  If it helps you to visualize it, imagine you're gently trying to rub something off that part of her vagina,  that's the general movement.  You can also pull your fingers out more and use more of combination of thrusting and pulling to give her more stimulation, I usually use a combination of the two.  

As she becomes more stimulated and excited, her G Spot will start to swell a bit and you'll be able to tell exactly where you need to massage. At this point, I'd suggest inserting either your index or ring finger depending on what angle is most comfortable for you (usually for me it's the ring finger)  and continue the same motion.  You should be able to tell that she's enjoying herself by now.  

As she gets closer to climaxing, you can take your free hand and gently push down on her belly, right above her pubic bone to increase the surface area of her G Spot and further help her along.  This is a bit tricky to describe, but imagine you are rubbing the palm of your exterior hand with the fingers that are inside of her.  You should be able to feel the pressure of your palm as you're massaging her.  I like to press with the outside edge of my palm and rub her clit with my thumb at this time.  

If she is going to squirt, the whole G Spot area will become engorged and swell significantly immediately before she does.  You'll need to stop for a second if that happens because I'm told it's much too sensitive to have direct stimulation.  It will return to normal after a second and you should definitely continue if she'll let you.  Don't worry or expect her to squirt though, it doesn't happen every time and some women are more inclined than others and some women will stop you before they do because it makes them uncomfortable.  Be alright with that, never push her to do something she doesn't want to do.

If you want to stimulate her G Spot with your penis, try the butterfly technique. In this position, you can push her legs forward to change the angle enough so that you're hitting her G spot with the shaft of your penis.  You can also get good stimuation with your penis in the flat doggy style (prone bone) position, but if you have a bigger penis sometimes this is too intense.  

Hope that helps!