Help with iMessages on iPhone and macbook?

I have been trying to get the messages from my iPhone onto my macbook.

When I have searched this is the past I have seen things telling me what to do, and I have tried to do all these things. I have fixed the preferences and settings on both my iPhone and Macbook, and have shut off both my laptop and iPhone, and restarted my wifi router.

Now, I tried to start a new message from my laptop, but every contact that I type in and try to message it gives me the message that whatever number or email I am trying to message "is not registered with iMessage". I know that they are because I can message them from my iPhone and other devices.

Does anyone know how to fix this? I would really like to be able to message from my laptop. Thank you for the help!