Most of the free versions of WordPress plugins are found on the plugins web page of WordPress.org. But, there are a lot many premium plugins of WordPress which needs to be purchased or used by paying periodically. Whatsoever be the kind of plugin you want to install. It can be installed in either of the following ways.
Download from Integrated Plugin Page in Admin Section
The easiest way to search and install a plugin in WordPress is to search it inside your WordPress installation and directly download it from there and activate it.
Go to Plugins –> Add New.
On this page, you will find featured, popular and recommended plugins. If you know what you are looking for you can search it too. Otherwise, you may either search it with the keyword or use tags below, if it fits your plugin search context. once you find it. Click Install Now.
After clicking Install Now, the plugin will be downloaded to your plugin folder and will be ready to be Activated. Means, Install Now button just download the plugin. But, if you wish to work with the plugin you need to activate it too.
So, to start using the newly downloaded plugin click on Activate button.
Install from a Zipped Plugin File.
Above method gives a way to download and install plugins available at WordPress.org. However, there are various plugins which are not uploaded on WordPress.org and still quite useful. Most of these plugins are premium or paid plugins.
In order to install such plugins. You need to first download them to your own server. The file would be compressed in zip format. Go to Plugins –> Add New –> Upload Plugins.
You will see a screen like above. Choose the zip file location and Click Install Now. After the plugin file is uploaded. It will give an option to active the plugin. Click Activate Plugin to activate.
If plugins are downloaded from Plugin page of WordPress.org directly, it would be downloaded in zip format. So, the 2nd method can also be used for plugins available at WordPress.org.
If you have any doubts or would like to share something, feel free to share it below.