How to Create a configuration File For MongoDB
The default path for brew installed mongodb on Mac OS X is /usr/local/etc/mongod.conf
Unless you have installed a packaged version of MongoDB (for example, using Homebrew or Mac Ports) you will have to create a config file manually, or just pass the appropriate command line parameters when starting up MongoDB.
If you want a commented example of a config file to start with, the mongodb.conf in the Debian/Ubuntu package should be a good starting point. Important options to check are the dbpath and logpath which will likely be different for you.
It would also be worth looking at the Homebrew mongodb formula which includes setting up a LaunchAgent script to manage the mongod service.
Yes, unless you install via a package manager (like apt or yum on Linux) you have to create this manually. Then, When you start mongod you simply need to specify where the config file is, for example:
./mongod -f /path/to/mongod.confFor how that file should look, just take a look here:
http://docs.mongodb.org/manual/reference/configuration-options/
You can also see the aforementioned Linux config files from the packages on github:
https://github.com/mongodb/mongo/blob/master/rpm/mongod.conf
https://github.com/mongodb/mongo/blob/master/debian/mongodb.conf