First see the section "Must-Have" Extensions.

Recommended Extensions (go to http://extensions.joomla.org ):

• The INTEGRATED Membership Site Quad:
  Community Builder adds user management and extends user profiles
  JACL allows you to create different groups
  AEC (Account Expiration Control) controls paid membership
  Fireboard is the community forum
• Forms
  ChronoForms is great, but does require some experience with html.
• MultiMedia
  AllVideos Plugin v2.4
• Ticket System/Project Management
  Taskhopper
• Business Directory
  Sigsiu Online Business Index2
• Extend Joomla!'s Menus
  Extended Menu - Download here (Requires some experience to configure)