You should not run mongod as root
How did you install mongodb?
If installed to start as service use sudo sysctl command
Check your installation instructions for exact command
If you want to start mongod manually try to run the command as normal user
If you run just mongod it tries start the instance on default port & default dirpath /data/db
If you want to start your mongod on another port & dirpath pass these parameters on command line
Ex:
mongod --port 28000 --dbpath yourdir --logpath etc
Check mongodb documentation for various command line params like fork etc