Well, the experts say make a "come hither" motion with your finger and that will actually hit it (it's on the top portion of the vaginal wall) but I've had that done to me and found it quite uncomfortable. What did work for me was simply pressing the finger upwards. Your girlfriend will feel the difference when you hit it, that's for sure...what technique will work is all dependent on her body and same goes for if she even likes the sensations. Sometimes they can be intense and that translates into uncomfortable pretty quick.
Rippled tissue located just behind the front wall of the vagina (towards the belly), about two inches inside. Stroking this area can greatly deepen a woman's sexual response.
good luck :)
Hey man, look up some diagrams as to where the g-spot is. Its basically located on the top of her vaginal wall, so if you place a finger inside her with your palm facing up while she's lying on her back, and feel on the top part of her vagina(the side closest to her belly), its usually feels like a rougher patch of skin than the rest of the vaginal skin. It also can feel like a bump when she's highly aroused. I have read that she is much more likely to respond to g-spot stimulation after she has experienced clitoral orgasm, so make sure she gets off from manual or oral contact first to get her 'primed' then go for it. Use a kinda of flat fingered come-here motion to stimulate the g-spot once you find it and put pressure on it as you stroke it. Make sure your fingernails are neatly trimmed so you don't scratch her up and make her sore. good luck