Well it depends on what kind of a relationship you want with the girl (friendship or romantic interest). But a generic way to start out for whatever kind of relationship you would like to have with her is a simple "hey how are you?" message. It's pretty casual and unless the girl already thinks that you're creepy, it won't scare her off and she's most likely reply to it.
Once she replies to you, you can tell her how you are and comment on her situation. If she says that she's not feeling so well, ask her what is wrong and that you hope she will feel better. Then afterwards, leave the conversation unless she seems enthusiastic to talk to you about her problems. Just keep in mind that getting to know someone who is not feeling well may not be the best time to get to know them. If she just says "good" or is terse about it, then ask her why she feels that way. Some girls can be pretty unresponsive and her reasons for feeling good is "just because...". If that happens, you should back off because she doesn't seem like she's in a mood for talking or that she doesn't want to talk to you. Either way, it's best to leave her be. If she is responsive, then continue talking about that subject and ask questions about her concerning that subject. For instance if she says that she feels good because she got a puppy, then ask her what kind of breed the puppy is and why she wanted a puppy. Ask her what she will name her puppy and who got her the puppy. But this will, of course, vary according to what subject you guys end up talking about.
If you just want to be friends, then just leave it at that. Casual small talk should do the trick of befriending others regardless of gender.
If you want to pursue something more such as a romantic interest, try flirting and joking a little bit. It's hard to tell whether someone is flirting or joking through text so be sure to put emoticons and whatnot in your messages to make sure she knows you aren't being totally serious. But if she somehow does take it seriously and freaks out, you should just apologize, saying that it was a joke.
After the initial steps, things will branch off depending on the people involved and the situation. No two text conversations are alike so I can't help you about the rest of it. But hopefully this will give you some sort of an idea of how to start things. Good luck!
