There are a plethora of free and paid fonts available. Once you get your hands on the font files, often times even experienced users tend to go clueless on installing their newly bought typographic asset. Here are the easiest ways to install fonts on the commonly used operating systems.

Mac + Linux (with font-viewer)
Select the .ttf files and double click → Follow the on-screen guide

Windows
Right click the .ttf files → Install

Linux (manually)
Copy the .ttf files to your fonts-directory (e.g., ~/.fonts) → Run sudo fc-cache

Linux using Font Manager

sudo apt-get install font-manager