The G-spot (yes, it exists!) is about two inches inside on the top (or front) wall of the vagina. You're aiming to stimulate the spongy tissue which surrounds the urethra via the vaginal wall. Firm, steady, back-and-forth pressure applied with a curved finger, dildo or vibrator (rather than a straight penis) is best. But don't expect automatic ecstasy when you finally hit it: for some it is pleasurable enough to elicit an orgasm, for others it can bring on female ejaculation, and still for others it's an annoying, even uncomfortable, sensation that's as likely to give them an orgasm as reading the US tax code.

See the Glam answer here.