If the plugin you want to install is available in the official plugin directory at https://wordpress.org/plugins/, then you can search for and install the plugin from within wp-admin. Just navigate to Plugins | Add New. You will find a search field in the top-right corner. Type in your desired plugin's name there (as shown in the following screenshot):
Once you see your plugin, click on the More Details link to see some more information about the features it has to offer:
I recommend you always look at this information carefully. Be sure to watch for version compatibility. There's a parameter labeled Compatible up to listed with the plugin's details. In some cases, you can risk installing a plugin that's a little outdated, but you should proceed with caution. It's also a good idea to have a glance at the number of downloads a plugin has had. The more downloads, the more people had the chance to take a look at the plugin and evaluate its quality.
After installation, you need to test the plugin carefully and verify that it's behaving correctly. Most of the time, if the Compatible up to parameter indicates an older version of WordPress, it doesn't necessarily mean that the plugin will fail to work with a newer version. It just means that it hasn't been thoroughly tested, hence the importance of performing your own tests. However, I strongly advise against installing any plugins that haven't been updated in more than two years. Luckily, whenever you encounter such a plugin, WordPress itself will either warn you through a message on the official plugin page, as shown in the following screenshot, or, inside the plugin details section in wp-admin, there will be a parameter labeled Last Updated:
If everything is fine, you can proceed by clicking on the Install Now button. After a couple of seconds, you will be able to activate the plugin. You'll see the Activate button that will appear. Consult with the following screenshot:
At this point, the plugin has been installed and activated. In other words, it should be fully operational.