New to MongoDB Can not run command mongo

After installing the MongoDB you should manually create a data folder.

By default MongoDB will store data in /data/db, 
but it won't automatically create that directory. To create it, do:

$ sudo mkdir -p /data/db/
$ sudo chown `id -u` /data/db

You can also tell MongoDB to use a different data directory,
with the --dbpath option.

For more detailed information go to MongoDB wiki page.