http://asktom.oracle.com/pls/ask/f?p=4950:8:::::F4950_P8_DISPLAYID:7490088329317 Check the link .
For Oracle 9i
select dbms_metadata.get_ddl( 'TABLE', 'EMP' ) from dual;
For Oracle 8 .. you have to use export , import utility to get the structure and its pain in ass .
I suggest you to use PLSQL Developer third party to make your job easy .