How to change Message account from email to phone

Hello all.

My girlfriend just got a Macbook Air and is just learning how to use the Message app.

It worked fine until today, when she started to message me. Her messages came from her email account instead of her phone number (we both have iPhones).

Does anyone know how she can change her settings so that when she sends a message it comes from her iPhone phone number and not her email? I've checked various threads on this site and haven't found a clear answer.

Thanks much.

H