How-to re-arrange blocks on the front-end page

Dear all,

A very dummy question : our page structure is the one you can see in the capture attached.
In particular, the right block is displayed : myConversations, myFriends, etc.

How-can I change this structure by, for example:
- removing the right block and stay on 2-columns
- replacing the right block by a many others right blocks: last Groups + last Articles + etc.

Thanks
Henri