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