localhost:port_number/em (what is my port number)?
the default port for 11g OEM is 1158. If it's not set to the default in your installation, then the value will be in portlist.ini. You can also find the full URL in the emd.properties file:

http://docs.oracle.com/cd/E25054_01/server.1111/e10897/em_manage.htm#BABHJAGE


  In your Web browser, enter the following URL:
  
  https://hostname:portnumber/em
  For example, if you installed the database on a host computer named comp42.example.com, and the installer indicated that your Enterprise Manager Console HTTP port number is 1158, then enter the following URL:
  
  https://comp42.example.com:1158/em
  If you have recently installed Oracle Database, you can determine the port number for Database Control on Linux and UNIX systems by viewing the $ORACLE_HOME/install/portlist.ini file.
  
  Otherwise, you can determine the port number for Database Control by searching for REPOSITORY_URL in the $ORACLE_HOME/Oracle_sid/sysman/config/emd.properties file, where Oracle_sid is the system identifier (SID) for your database instance.


$ORACLE_HOME is the home directory you chose when installing the database.