You can download and install BuddyPress using the built in WordPress plugin installer. If you download BuddyPress manually, make sure it is uploaded to "/wp-content/plugins/buddypress/".
Activate BuddyPress in the "Plugins" admin panel using the "Activate" link. You'll then see a message asking you to complete the BuddyPress Installation Wizard, which will guide you through configuring your site for BuddyPress.
--- Discussion Forums ---
BuddyPress includes full support for discussion forums. Each group created on your site can have its own forum. If you'd like to enable this feature, after completing the Installation Wizard, visit the "Forums Setup" item under the "BuddyPress" menu in your WordPress admin area, and follow the on-screen instructions.
Of course! Once you've installed and activated BuddyPress, the Installation Wizard will guide you through the available theme options. You'll be given the option of installing the BuddyPress Template Pack, which will help you add BuddyPress compatibility to your existing theme.
Be sure to also try out the default theme bundled with BuddyPress. It provides all the awesome features of a standard WordPress blog, but also integrates BuddyPress's features in a seamless and beautiful way. The BuddyPress Default theme is a snap to customize, with full support for custom headers and backgrounds and multiple widget areas. It also makes a great starting point for your own child theme.
Yes! If your WordPress site has multisite enabled, BuddyPress will support the global tracking of blogs, posts and comments.
The support forums can be found at http://buddypress.org/forums/.
The documentation codex can be found at http://codex.buddypress.org/.
Report bugs and participate in development at http://buddypress.trac.wordpress.org.
Check out the development trunk of BuddyPress via Subversion, from http://buddypress.svn.wordpress.org/trunk/
See http://codex.buddypress.org/releases/version-1-5/ for 1.5. See http://buddypress.org/about/release-history/ for all other versions.