Don't tell her in a text message, via Facebook chat or in an email. This is the kind of thing you should talk about face to face.
Tell her somewhere private and relaxed. At one of your homes when there aren't people around to interrupt is a good option.
Keep it simple and stick to the facts. There is no need to feel guilty or ashamed that you have an STD, lots of people have them. Tell her what your symptoms are, how you treat it, what you know about transmission and how to protect her from getting it too.
What to SayHere are a few ways you might want to start the conversation: "Honey, there's something about me I think you should know..."
"I need to tell you something, but I'm afraid of how you're going to react..."
"Before we get intimate, there's something we need to discuss..."
What to expectThe range of reactions from your partner can vary widely. It probably took you some time to accept the fact that you had an STD. It might take your partner time to come to terms with it too. Some people will have a lot of questions. Some people might not be prepared and may break up with you. Some partners may need to step away for a while and gather information and to decide if they are okay being in a relationship with someone with a STD. Others will just roll with it and get excited about finding new ways to eroticize safer sex. Listen to your partner and try to accept where they are, but you don't have to take abuse if she gets angry at you.
She might want to know how you contacted the STD and who you got it from. If you can, try to avoid "outing" the person who you contacted the STD from. Just as you wouldn't want others to spread it around that you have an STD, the person you got it from doesn't want that spread about her or him either.
Regardless of how you think she might react, it is important that you tell your partner about your STD. She deserves to know.