Your heart may race the second your crush signs on, but that doesn't mean you have to race to start your conversation. It's better to play it cool and look like you're not just hanging around waiting for the second he or she signs on.  Unless your crush is the kind of person who only signs on for a minute at a time, let some time pass--at least 10 or 15 minutes. Then your crush will see that while you want to continue your conversation, you have other things to do online. This will make you look less needy or clingy. Don't wait too long or your crush won't think you're interested. When you do greet your crush, it's important to start the conversation on the right foot so your crush wants to spend more time talking to you. The key here is to play it cool---but not too cool.  Saying, "How are you?" or "What's up?" is perfectly acceptable. Don't try too hard to wow your crush with your opener. You can start with an interesting story about something that happened to you, something you were previously talking about, or something cool you just heard on the news. Again, keep it short and don't try too hard. It's important to strike a balance in your relationship. You don't want to be the one who always sends the first message, because there might be a chance that your crush may not be all that eager to talk to you after all. It may be the case that your crush is just shy and likes it when you make all the moves. But if this is the case, then it could be a sign that if you take your relationship to the next level, your crush will still expect you to do all the work. That could get annoying really fast.

TL;DR in Vietnamese:
Đừng nhắn tin cho người ấy ngay giây phút mà họ lên mạng. Bắt chuyện một cách thú vị và tự nhiên. Đừng lúc nào cũng là người nói "xin chào" trước.