Editing messages on registration page

On my registration page when a person is creating an account the first field is User which is ambiguous for many people, as they don't know if this is going to be the name that shows or what it is.

Firstly I would like to put some explanation above it.

Another thing is that there is no where that tells a person that it must be lower case with no spaces or special characters. And if they use upper case or put a space the error only tells one that it has to be lower case - nothing about special characters or spaces.

I would like to make that error message more informative.

How do I do this?