After you get over the urge to run and hide, lock your teen in the bathroom, shave his or her head, and save yourself, take a deep breath and think about what is important here. You are likely panicked because you know that sooner or later someone will break your teens heart - and there is nothing you can do about it, or is there?
Talk to your teen and share what you are feeling as well as what you know. Being new to the world of love/lust/hormones, there are some really great conversations to be had now about balance, friendship, and healthy relationships! First, your teen may be overwhelmed with how wonderful it feels to be in love and you can help remind your teen about balance, and the importance of not losing themselves for love. Your teen needs to stay "true to self" instead of becoming an appendage to the new love. Encourage your teen to stay connected to friends, school, outside activities, family, and sports, while making room for the new love.
You might mention that if that becomes an issue, you can help by setting limits on the amount of hanging out at home, phone, text, and computer time, to help her learn to balance life and love/lust/hormones. This is not a threat - just a supportive way to help your teen transition in the world of love!
Together you can set the expectations that honor this new part of life, make your teen feel listened to and involved with the new contract - the new couple spends time with the family, grades stay up, activities continue, chores, whatever else her life includes must all continue - because your teen has to be a "person" first before a girlfriend or boyfriend. The We're Talking web site has a great section called the abcs of healthy relationships, which will provide many reminders about knowing when a relationship is not healthy.
Along those same lines, it is important to talk about the importance of friendship - and how you want the first few months together to be spent with family - because early in relationships the goal is to learn to trust each other, find things that you have in common, and become parts of each others lives. Friendship is stronger in the long run than hormones - and if either member of the couple is motivated by anything else other than love - s/he will not make it through the "getting to know all about you" phase.
P.S. Remember that the greater the age difference, and the more time alone they share, the more likely teens will take new love to sexual realms, so be aware and good luck!