First, my own daughter is 2 so the only experience I have with this is from when i was a teenager.
I think you should start by having your daughter talk about it first - why did she make such an important decision? Is she happy about it? Did she feel ready? Does she know about contraception? STDs? My guess is that, at 13, she is NOT totally pleased with herself or her decision and will be more likely to admit it if you talk empathetically with her than if you go in guns blazing.
From there, I would share YOUR point of view - all the reasons why its not a great idea for teens to be having sex - from pregnancy and STD risk, to the need to learn how to deal with emotional relationships first, before introducing a physical relationships, and, finally, to the fact that teenagers dont need to be in serious, long term relationships to begin with - that they should focus on schools and friends, and activities. Spending all your time wrapped up in your boyfriends takes away from all of those other important experiences.
Yelling and crying will not make her stop having sex, but it will make her stop talking about it, and other important issues, with you.
You can let her know that you think its a very very poor and damaging choice for her, without making her feel terrible and like you hate her.
Tell her you love her and she is wonderful and beautiful and that you want her to enjoy her teen years without the problems brought on by sexual relationships.