Well, it all depends on the person. Remember that some woman can't even orgasm through g-spot stimulation. However, I'd recommend using sheets that you are ready to clean quick, or putting something under you. It can get VERY messy.
And as for learning how to find your g-spot and have an orgasm, there's a great article over at scareleteen.com. It's near the bottom of the arcticle, right after "Life on the Inside," however, I suggest reading the whole thing!
http://www.scarleteen.com/article/bo...sexual_anatomy