How do I stimulate a girl?

Hi Jack, first of all you need to make her wet and turned on before you start to finger her. Start by just kissing her as normal then work your way down to the side of her neck just below her ear and kiss around that area while running your hands over her body, start to stroke her breasts through her cloths for a while continuing to kiss her then you can work your way down her and with your hands be down to her inner thighs and stroke them avoiding her vagina just yet, she should be starting to show signs now of getting turned on. Work your hand up to her vagina and stroke the outside of her lips to start with through her underwear then slowly slip your hand inside her underwear and start to stimulate her clitoris, you should be able to give her an orgasm by doing that, then you can slide you middle finger between her lips and slowly slipping it inside her but make sure your finger is yet first. :-)