To give additional functionality to your favourite editor you need to install additional packages. The steps to install packages in Atom are given below. Following steps all applicable irrespective of the operating system.
Type cmd + , (on a Mac) or ctrl + , (on a PC) which brings up the Settings View
Click on the Install tab.
Type the package name you want to install into the Search packages input field and press the Enter Key.
Press the Install button on the package you wish to install.
Settings View can also be found by going to Atom > Preferences (Mac), File > Settings (PC) or Edit > Preferences (Linux).
You can also employ the Command Palette to directly access the Install tab of Settings View by typing cmd + shift + p (Mac) or ctrl + shift + p (PC), then type Install packages and select Settings View:Install Packages and Themes.
Mention any further queries in the comments below. If you need a guide for installing Atom on Linux, you can find my post here. A detailed post about ten best plugins to get you started with Atom are coming soon.