please tell me which one sentence is correct or better ?
If there is anything you want to know about China ,leave me a message or contact me.( Anything you want to know,you can leave me a message or Anything you want to know,leave me a message--Which one is right or better?)

Question

Answer

They are all correct. the first is the most complete, but Natives will often abbreviate as you did in your last example.

If you need anything, you can message me. If you need anything, message me. If you need anything, send me a message.