I like a girl now, and I'm trying to catch up with her, but I don't know what to say when I send her a text message to chat about QQ every day.

You can learn about her hobbies! You can also make her know you better. If she prefers to study, you can also go to a classroom with her to study by yourself! You can do something substantial for her, or you can tell her something about your life: for example, ask her to help you analyze what you have encountered (not about you two, of course), so that you can get to know her better. If there is an opportunity, you can ask her out, or if she has something to do, you can go with her.

The most important thing is to let her see your sincerity, but don't ask people if they want to be your girlfriend at the beginning. This is false, because you don't know each other yet.

Wish you success!