Do you know where the g-spot is? You should! It is, after all, what allows some women to orgasm during vaginal penetration. Find her sweet spot manually and she'll likely want to jump on your cock for more, so this is really a good thing to know.
If your finger is inside a girl, the g-spot is your target. Why? Because it stimulates her clitoris from the inside. Seriously, no gal past the age of fifteen should have to deal with some inept schmuck shoving a finger or two straight in and out of her pussy - it feels no better than being poked it the armpit.
You'll know the g-spot by its spongey texture and, hopefully, a reaction from your lady friend. Many of the girls I sleep with have never been with a woman before and are amazed at the intensity of having their g-spot manually stimulated.
If you're going for the g-spot in the sack, the most likely position is to have the gal on her back with some pillows elevating her hips and her legs over the guy's shoulders. This allows for deep penetration, so it shouldn't be your first position. Work up to it. Another to try is bending over something like a couch, where her hips are higher than her torso and doing it from behind.
Want more g-spot info? Take a peek at Violet Blue's g-spot page.