BuddyPress.org

General theming guidance is beyond the scope of these forums as it really is more an issue of learning how to use CSS and WordPress/BuddyPress template loops to create the design you are after.

But, to help get you started, here a few links:

1. Advice on using the default BP themes or Skeleton Theme that comes with BP
2. A great tool to use while designing your theme.
3. Basic how tos on WordPress theme design. NOTE: this link pertains to wordpress themes in general, not BuddyPress themes. But, you should really understand WP theming before you set out to do a BP custom theme.