The G-spot is a ring of spongy erectile tissue surrounding the urethra (where urine leaves the body), located around one to two inches inside the vaginal opening, and is on the front wall of the vagina (the front wall being the "top," or the direction of the front of your body). Some women really like G-spot stimulation, which you can do with your fingers when you go down on her, but many others don't find it enjoyable. Ask her if she likes to have her G-spot stimulated, and if she does, only do it when she's really turned on. When you try it, insert your finger (or fingers) and give a sort of "come hither" stroke along the front wall of her vagina. When aroused, it will feel firm to the touch.

Referrence:G-spot(Chinese Version)