If it is your first time, I wouldn't worry too much about the finer points of female anatomy just yet. Concentrate on making sure your partner is nice and relaxed, take your time, respond to her signals, which may not be verbal, and be patient. If the lady knows where her G spot is - and most do - she will direct you to it. It is located just inside the vagina at the front, but it is not a physical 'spot', more an errogenous zone. Try to concentrate on your partner's body as a whole, it's making love, not target practice.