Step by step instruction to install MongoDB on Ubuntu based on: http://docs.mongodb.org/manual/tutorial/install-mongodb-on-ubuntu/
Configure Package Management System (APT)
The Ubuntu package management tool (i.e. dpkg and apt) ensure package consistency and authenticity by requiring that distributors sign packages with GPG keys. Issue the following command to import theMongoDB public GPG Key:
sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 7F0CEB10
Create a /etc/apt/sources.list.d/mongodb.list file using the following command.
echo 'deb http://downloads-distro.mongodb.org/repo/ubuntu-upstart dist 10gen' | sudo tee /etc/apt/sources.list.d/mongodb.list
Now issue the following command to reload your repository:
sudo apt-get update
Issue the following command to install the latest stable version of MongoDB:
sudo apt-get install mongodb-10gen
When this command completes, you have successfully installed MongoDB! Continue for configuration and start-up suggestions.