BuddyPress.org

First of all you’ll want to get set up with an installation of WPMU and make sure that is running correctly.

It’s probably a good idea to head over to the WPMU forums where there are a lot of expect WPMU users (https://mu.wordpress.org/forums). They will be able to help you get started with MU.

Once you have that up and running, come back here and we can guide you through installing BuddyPress on top.