If this is your first time having sex or making love to a woman or having sex in general, be assured that it is normal to be nervous.Make sure that before you try to turn another woman on, you know your own body first. It helps if you know what excites you. (For this you WILL have to masturbate ) :)
Giving a woman an orgasm (or just making them feel good sometimes could be the greatest turn-on. So you want to know what is going to make her feel good, and like I said the only way for you to know that is for you to know yourself first.
Finding the G Spot
If you want to find the g-spot (the happy spot), simply insert your middle finger in the vagina as far as it will go, without hurting your partner of course. Then move your finger in a "come hither" motion touching the top of the vagina until u find an area that is slightly rougher than the rest of the vagina, and ball-shaped. If she feels good when you rub it, then you have succeeded young grasshopper.
Make sure to read up on safe sex for lesbians. Yes std's also apply to lesbians.
Fantasize
what do you want to do to her, what do you want her to do to you? You gotta imagine before you can do right?
Toys are always fun but some flesh on flesh is the best, really get to know her body and make her come in different ways, tease her A LOT (she will love it).
Go slow, touch her all over, kiss every part of her body that you undress as you undress her. Nibble her ear, kiss her inner thighs and caress them. Massage her Butt, and slowly suck on her fingers. Tease her - Caress her body, her breasts, especially around her pubic area, tease her clit, but don't go in until she cant take it anymore and starts begging you to do her.

And when she least expects it, penetrate her with two fingers, slowly at first and then pick up speed. Lick her clit while you are doing her and with your free hand caress her breasts and her sides and her stomach. Let her hold onto your hand if she needs to (this would be a very good sign).
Moaning on your part is a plus, it will turn her on.
Well there you have it, now find a woman and put my advise to use :):):)