Multisite confussion

Hi and thanks for reading.

I am trying to set MU, and I got this
ERROR: Your WordPress address must match your Site address before creating a Network.

I did some research and I understand what i need to do, but in the same research i got somehow confused.

this is my site
http://myname.com

this is my WP address
http://myname.com/hereWP

I have other folders like
http://myname.com/PG
that have content that is point to other domains I manage.

I understand that if i move the content of /hereWP to the root, then my Error will be solved? No need to do anything else?

I want to use MU so that I can design different sites and blogs for different domains, that might be hosted on my same server or others. For doing that do I have to then install and run the WordPress MU Domain Mapping Pluggin?

Thank you for any help or advice.