Ok, I will explain as simply as possible
A GPU (graphics processing unit) is a device that does the processing of the graphics/video/display whereas the cpu (central processing unit) does the processing of the numbers. A GPU is a must in order to have any kind of display output.
Some CPUs have an integrated GPU that is not very high standard but is able to display to at least a single 720p monitor (nowadays).
The FX8320 doesn't have an integrated GPU so if you install it without one, you will not be able to boot your system and will get an error indication light for no graphics. Hence, you need a GPU along side the CPU to have a full system.
What GPU to get is a big question. They range in price, performance, size, age etc. If you give your intended use of the computer we can surely advise which GPU to get for your budget.
For how to install it...instead of me explaining, its best if you watch
https://www.youtube.com/watch?v=O9x097QRXeA
Also just see this if you are interested https://www.youtube.com/watch?v=c3BKuNXdM5A