hi am working fo civil3d but i cant save from 2012 version to 2011

That's right, you can't.The current file format is 2010 which covers releases 2010, 2011 and 2012.  A new file format will probably be available when 2013 is released.

is there somebody can teach me how to use civil 3d..many thanks...

Have you checked online for any tutorials?Do you have a good after-market book about Civil 3D?Have you consulted your User's Guide?