There are two ways to install a plugin in WordPress.
The first step is to navigate to the Plugins menu section in your WordPress dashboard.
Hover over Plugins, then go to Add New.
This allows you to search for a plugin you are looking for. If it's available in the WordPress repository, you can search for it and it will allow you to install it from there.
For example, if you want to install Yoast, you can type "yoast" into the keyword search bar on the Add New page. You don't use quotes. You will see the Yoast SEO plugin, where you can click the Install button, then hit Activate once installed.
If you have a premium plugin or one that is not offered on the WordPress repository, you still hover over Plugins > Add New
Hit the Upload Plugin button and it will then ask you for a file. This HAS TO BE a .zip file. It can't just be a simple file. Plugin creators should provide you with a zip file.
You will choose the file, then hit Install Now, then Activate on the next page.
Your plugin is now active and ready to use.