The G-Spot is easy to find. Although, according to most women, is over rated. About finger length in, and up. With her on her back as an example. You really can't miss it as it is firm and of a different texture from the rest of the vaginal tissue. Massaging it will stimulate the paraprostatic (sp..I'm drunk) gland. She'll say she feels like she want's to urinate.That's where you want to be. Now it's up to her. Can she over come her reluctance and inhibition about possibly urinating during this experience? Usually not. At least initially.
With practice, it is said that women can have the best orgasm they have ever had. In most cases, you're better off with direct clitoral stimulation.
Have fun.