You can install packages on your Ubuntu in many ways. One of them is by using PPA (Personal Package Archive).
If you want to add a new PPA repository you will need to use command such as
$ sudo add-apt-repository ppa:ondrej/nginx-mainline
Sometimes on a new fresh installed system, you will get error such as
sudo: add-apt-repository: command not found
What this means is, you don’t have the required package to run that command.
The solution is so easy
$ sudo apt-get install software-properties-common
And you are good to go.