be gentle, be caring, make sure you get her nice and wet before you insert your finger into the vagina. play with the clit. when ready get a finger or two in there and explore. be attentive to her body language, for any movement that indicates pleasure you know where she likes it. stay there for awhile, move around and tease that spot. that should drive her crazy. move around and find other spots. do some research on the g-spot, google it. going there will pleasure many, many women.
but the main thing to remember is to be gentle and attentive...