Hi, I’m a beginner.
I’ve created an armature with all the associated vertex groups. It is a human. I’ve created its hair separately and I want that when I move the head bone, it moves the hair too.
How can I?
Make sure you are in pose mode, select the mesh for the hair and then select your head bone and hit CTRL+P and set the parent to the bone 
It worked, thanks!
You’re welcome! Glad it did