Don't Make Small Talk
If you've just started talking to a girl, you don't need a boring image of yourself in her head. Don't go with a typical message like "What are you up to?" usually, the message will just be 'nothing much, you?' and after that, the conversation will be duller than Dwight Shrude's wardrobe. Ask her a question that she is going to have an answer to and build off the rest of the conversation off that.
Head into the Conversation With a Purpose
If you've known the girl for a bit, for instance, if she's a colleague or goes to the same school/college, it's a good idea to head in with a topic in mind. Your intention is to talk and keep her talking. The best way to do that is to give her a purpose she can talk about. Ask for help on a certain project you're working on, discuss co-workers and classmates. However, it is important to know that you have to convert the conversation into a casual one quickly. Otherwise, there's a good chance that any conversation you start with her will be super formal.
Text Her Fast
Don't go for the old "text her after three days" regime that we so commonly hear in movies and teen TV shows. If you get a girl's number, it's incredibly important to text her within the first 24 hours because that is what she is expecting from you. If you cross that 24-hour barrier, she'll start thinking that you're not interested in her.
Talk Properly
There's nothing more unattractive than watching a guy write write 'y' instead of 'why' just because it saves time. It might just be the way you type, but this forwards the image that you don't have the time or won't take the time to talk to the girl. This is a major turn off and doesn't be surprised if the message you receive is dull.
Ask a Question
When you succeed at getting her number, the next step is sending her a text. Say hi, or hey to let her know it’s you. But don’t just hi, because it looks very uninteresting. The best way to start a conversation is to ask an open-ended question so that even if she tries hard enough to answer it with a yes or no, she has to add some detail, which starts the conversation. 
