how to prevent any update to my database before a exp operation?
From the command line 


from the command line enter lsnrctl and then stop (stops the listener, only connections from the server possible)
connect to sqlplus as sysdba
shutdown immediate  ( makes sure all sessions are terminated)
startup restrict  (only allows users with dba privilege to log on)
as Mindaugas Riauba mentions disable all jobs likely to change data during the export
export your data
alter system disable restricted session
back to command line enter lsnrctl and start