If you're running Linux, here's a guide...

If you're on an Ubuntu variant for development, here's a great guide to get it installed:

https://docs.mongodb.com/v3.0/tutorial/install-mongodb-on-ubuntu/

There are instructions there for other distributions and package managers, as well. Simply cut and paste the commands to add the repo to your package manager and the proceed to the next steps.

Steps in CLI

sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10

echo "deb http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list

sudo apt-get update

sudo apt-get install -y mongodb-org

Don't worry about the documented run/start of Mongo, as another way is made available below.

Alternate Run/Start Method

For an Ubuntu variant using the CLI to install, create the default directory for MongoDB in root system directory:

sudo mkdir -p /data/db

This creates the needed directory for Mong and adds it as a path to issue the mongod command soon. And then issue the following command from the CLI and substitute username with your own system username that you're using on Linux:

sudo chown -R username /data/db

This will give you (the user) permission to use that folder so that Mongo doesn't throw an error, stating that it either can't find it, write to it, or access it. Once that's done, then issue the final command:

mongod

You should see a multiple line confirmation that the Mongo service has started and the CLI window should just remain open for it to continue to listen for connections. If you're using a JetBrains product, you can also start this from the CLI within the development environment and simply leave that CLI instance open and add another if needed for routine tasks.