Spread her pussy lips open with the fingers of one hand, and start licking from bottom to top on the outside of her pussy. Very slowly at first, and with each lick you push your face a little further into her box, licking a little faster with each stroke. With your free hand reach up and grasp one of her nipples firmly between your thumb and forefinger and apply gentle pressure, twisting slightly and increasing the pressure, but not too hard, else it might become too painful. Let her tell you if it hurts too much and ease it back.
If you have long fingers, you can also gently probe around her asshole with the hand you're using to spread her pussylips. Do NOT use those same fingers to put them into her pussy as you can cause the spread of bacteria, unless she's been cleaned down there.
As you work your tongue into her hole, when you near a top stroke, you'll feel a hard bump which is her clit. It becomes engorged with blood like our cocks do when we're excited, thus increasing sensory perceptions.
As she starts to push her pussy toward your mouth more and more, try to GENTLY grasp the clit between your lips and run your tongue over it. She should start bucking her hips by that time and you can alternate between her tits and pussy and asshole.
She will start getting very wet by this time and you can really let yourself go and eat that girl til she's writhing all over the bed.
Do not rush this as girls take a good bit longer to get excited sexually than us guys.
Take your time, be gentle, and let her guide you to what makes her feel good. If you do that, she is sure to reciprocate and make you feel good.