Communication is key. Many of us think sexual pleasure should be automatic and if we have to ask for what we want, we must not be "doing it right." But asking our lover to do something to give us pleasure can be very exciting! Start out by murmuring or moaning when you like what she is doing, and she will do the same; then you can work up to specifics like "That feels good" or "Don't stop" or "Slower" or "I'm coming!"
 
For some, having a few drinks or drugs will help lower your inhibitions and make you less nervous and more bold. But since this is your first time you also want to remember things with a clear head, and not be led into something you might regret doing or saying. So do yourself a favor and keep consumption of alcohol or recreational substances to a minimum.
 
Concerned about STDS or AIDS? These are certainly a risk among lesbians as well as other populations. Read up on safe sex tips for lesbians online before you head into your first encounter, and you will at least be informed enough to know how to deal with what comes up. Don't be afraid to ask your partner about various activities or her medical history if you are concerned. It might disrupt the mood at first, or it might relax you enough that you can proceed confidently. But it's important to be aware of the risks.
 
Some lesbians like to use toys or role-playing, or maybe S&M or bondage. Nothing wrong with spicing things up, but for your first encounter, try to just go for a flesh-only encounter.  There will plenty of time to experiment and add accessories, but for now you will both be enjoying the newness of each other.
 
What if you freeze up and don't know what to do? Don't worry. Relax. Start with a backrub or foot massage. Stay on the sofa kissing with your clothes on for a while. It's not all about how you are going to make her come, or vice versa. Explore her body, see how she likes to be kissed or touched. Ask her what she would like you to do. Listen to her breathing and moaning, ask if she likes what you're doing and wants you to continue.
 
But wait, what if she asks for something you're not ready to do? If you have gotten this far, she knows it is your first time (and if you did not tell her that, well, it's too late now!) Tell her you feel a bit awkward doing something you have not done before. If you are game to try it though, ask her to help you. She may be fine with being manually stimulated to orgasm, if you are not ready to go down on her. She may be willing to save fisting for another time if you want more time trying other things. Likewise, let her know what you are comfortable with. Making you come might be enough for her, or vice versa. The whole point is to get to know this person's body and their desires. Even if you never have sex with her again, this will have been a very special experience for you.
 
Maybe you will spend the night with her, or she with you. That leaves a night of snuggling ahead and maybe waking up to discover her body in the sunlight. Maybe you will be energized to explore some more! Maybe you will awaken feeling awkward. Whatever you're feeling is fine, women are pretty good at sensing what is up so talking about it is a good idea. Or maybe you just want to ask  if you can hold each other.
 
If you decide to see each other again, imagine how much more comfortable you'll be! And how much fantasizing you can do about her in the meantime, and she about you. If you decide you want to continue your exploration into lesbian sex, this "first time" will be part of your journey.