Starting Oracle Database Enterprise Manager

1. Switch user to root
    su root
     or
    sudo su

2. Set ORACLE_SID
    ORACLE_SID = ORCL
    export ORACLE_SID

3. Set ORACLE_HOSTNAME
    ORACLE_HOSTNAME= localhost
    export ORACLE_HOSTNAME

4. emctl start dbconsole

    emctl is available in bin folder in Oracle  installation folder
    oracle/product/11.2.0/dbhome_1/bin

5. Access oracle enterprise manager using the URL: https://localhost:1158/em