Moodle has a great and simple system for installing 3rd party plugins. In most cases you will find the "Install plugins" page at : 

Site administration -> plugins -> install plugins

On the boost theme, that looks like this:

On the clean theme, that looks like this:

If you still can not find it, you can access directly by making the following URL from your site URL.

[Moodle Site URL]/admin/tool/installaddon/index.php