Hello Vinay,
There are lot of demo programs available in SCN and it can be achieved using the function module
REUSE_ALV_COMMENTARY_WRITE
http://wiki.scn.sap.com/wiki/display/ABAP/Add+Header+to+ALV+report
In SE80 package SLIS check for the programs starting with BCALV_TEST_FULLSCREEN* which explains you in detailed about the header.
Regards,
Thanga
