Hi,
Use Class: cl_abap_typedescr
Here's some examples:
http://jumpifzero.wordpress.com/2011/08/25/dynamically-determining-the-fields-of-an-abap-structure/
http://help.sap.com/saphelp_nw2004s/helpdata/en/fc/eb3145358411d1829f0000e829fbfe/content.htm
Or
if you already know the fieldnames you can use field symbols you can use syntax "ASSIGN COMPONENT.. "
http://help.sap.com/saphelp_nw04/Helpdata/EN/fc/eb3923358411d1829f0000e829fbfe/content.htm
Br,
Andrei