[voice]
...Well, first of all, you will be a lot older. I'm sure you know that.
But -- no matter what, you should make sure it's someone you care for, and someone who cares for you. Someone you trust. Especially the first time.
Sex can be a natural part of a relationship -- a way of expressing love, commitment. But you should know that it isn't the only way to express love, or the best way. If someone you care about ever starts pressuring you to have sex, you shouldn't feel like you need to do that. Sex is a part of relationships, but it isn't the whole, and it doesn't automatically make you closer to another person, or... make them care more about you.