The first sign of excitement is vaginal lubrication. Within 10 to 30 seconds your vagina becomes wet. The walls become covered with a colourless fluid. It is a natural lubricant, which makes fingering and intercourse more pleasant.
The vagina lengthens and distends. The first third of the vagina becomes a little wider, but the middle and top really expand (see also: orgasmic platform).
The entrance of the vagina is most sensitive to touch, while the top is more sensitive to pressure.
If you stimulate the vagina with fingers or a dildo it is best to concentrate on the front wall, because it is more sensitive than the back wall. That’s why women in the missionary position often tip up their hips. This increases the chances that the penis will touch the front wall. Some women have a very sensitive spot at the front wall, one or two 2 inches above the entrance: the G-spot.
Know that the vagina is far less sensitive than the rest of the vulva. That’s why most girls don’t climax during intercourse. In order to orgasm you’ll probably need (some) clitoral stimulation.