For both partners:
How do I have "good" sex?
What follows is not a set of rules. Rules about sex are impossible-- what should matter is that what you do makes you feel good. And "feeling good" should last past the sex itself-- you should not feel anxious afterwards about getting her pregnant or catching some horrible disease, so planning ahead about contraception and safe sex is part of the idea.
What time should we have sex?
When you have sex doesn't really matter. What should matter instead is that you and your partner have sex when you are both comfortable. Some people prefer to make love at the break of dawn, some in the afternoon sun, some in the darkness of night. More important than time of day is the time you have to spend. Give yourself a lot of time to have sex the first time. A weekend is ideal, but at least the whole day, including sleep time.
Should we eat or drink anything before sex?
Avoid eating a heavy meal, since that'll just make you sleepy. Eat light, don't drink too much alcohol, if any at all. It may help you shed inhibitions, but it may also make his erection much more difficult to achieve and you want to spend more time in the bedroom than the bathroom anyway, right?
Where should we have sex?
Where you have sex is probably a more important decision. Finding a place where you both can be private for up to forty-eight hours can be difficult at that age where most people are planning on losing their virginity. But it's worth it. Cars are no longer big enough to have sex in, and the outdoors has less privacy, bugs, sand, and pine needles.
A bed is probably the best thing to make love in, still. Hopefully you'll have a room to yourself, with all the amenities that a bedroom affords, including heat, comfort, space, and the bathroom. Take a shower together! If you're about to have sex, you've probably had your hands all over each other before now. Get to know each other's bodies. Take your time. That's what lovemaking is about.
What do we need to bring?
Bring what you need to make you comfortable. Birth control, condoms, maybe your favorite pillow or a bathrobe.
For her:
What do I do first?
You can't expect him to know what makes you feel good. You'll have to tell him or show him, and that may mean taking some of the initiative, taking his hands and placing them where they make you feel good. Go slow. If it's his first time, he may well be totally nervous about what you're about to do, and his penis may not respond at first. Patience, gentleness, and understanding are required to bring it back to life, and that may be hard for you to achieve, but that's why we told you to give yourselves lots of time.
Will it hurt?
You have probably heard horror stories about how much losing your virginity hurts. For a few women, it does, but with the right touch and the right partner, you should be able to take his penis into your body without pain. Have him take his time, use a lubricant, and press his fingers into you, opening you up slowly. Tell him when it feels good and when it hurts.
What position should I use?
Many women prefer to have sex the first time being on top, where they can control the first entry. Others want to be on the bottom and give their lovers that control. Choose what's best for you. Just remember to tell him to go slow, take your time, and if you feel the need, use a commercial lubricant like KY Jelly.
For him:
What if I can't get it up?
It may sound funny, but your penis, which has worked great for years, may suddenly go on strike at your first chance at "real" sex. That's natural-- you're nervous. Take a deep breath. Do something else for a while with your hands, your lips and your tongue. Try to forget about your anxiety, and your penis will respond. It's only a temporary thing.
Should I tell her if I'm a virgin?
Many men think that because they're men, they should be in charge of the sex, regardless of who has the more experience. If you're a virgin and she's not, tell her, and let her lead if she wishes. This is as much a learning experience as a loving one. Don't be afraid to confess the truth. A lot of women would rather know that your fumbling is inexperience, rather than just sheer ineptitude, and will gratefully show you the ways of the world.
Will we come at the same time?
Don't worry about making orgasm simultaneous, either. Some women do not orgasm during intercourse, and even if your girlfriend is capable of climax, the odds are very much against you coming at the same time. Enjoy yourself, and rely on her to tell you the truth when she's enjoying herself.
What if I orgasm too soon?
If you actually climax much too soon before you wanted to, take your time, take a nap, and try again. The second time you should be much more relaxed and ready to take your time-- so will your penis.
Am I big enough? Too big? The right shape?
Another common concern is size. The average penis is slightly more than five and a half inches in length when erect, and that's more than enough to hit every major nerve center in the vagina, the legendary G-spot included. The vagina is capable of stretching to take a large penis, or shape itself to pleasure a small one. Size has very little to do with your ability as a lover.
Another common issue is shape. Some men become concerned because their penis bends downwards, or to the left, and assume that because they never see men like them in erotic movies that they're not normal. Others worry that a downward bend will make sex difficult or painful because the vagina isn't shaped with that bend in mind.
Keep in mind that sex can be performed in any number of positions. The penis and vagina can be matched in many different ways, and each new position can bring new pleasures to you and your partner. Some people believe that a downward-bending penis is much easier to perform oral sex upon.
Will I be a good lover?
Being a good lover doesn't happen automatically. With the right partner, time, care, and practice, you have everything you need to become a great lover. Your first times, for both you, will be fumbling and awkward, but hopefully they'll be the start of great times for the rest of your lives.