To install a WordPress theme or plugin, you can follow these steps:

Log in to your WordPress dashboard.
Go to the "Appearance" or "Plugins" menu, depending on what you want to install.
Click on the "Add New" button.
If you're installing a theme, you can either search for a theme in the WordPress repository or upload a theme from your computer. If you're installing a plugin, you can either search for a plugin in the WordPress repository or upload a plugin from your computer.
Once you've found the theme or plugin you want to install, click on the "Install Now" button.
After the installation is complete, click on the "Activate" button to activate the theme or plugin.

As for the common and typical WordPress themes and plugins, here are a few examples:

Themes:

Twenty Twenty-One: this is the default WordPress theme that comes with every installation.
Astra: a lightweight and customizable theme that's popular with bloggers and business owners.
Divi: a popular multipurpose theme with a drag-and-drop page builder.

Plugins:

Yoast SEO: a plugin that helps you optimize your content for search engines.
Jetpack: a plugin that adds a suite of features to your site, including security, performance, and analytics.
Contact Form 7: a plugin that allows you to create and manage multiple contact forms.

These are just a few examples, as there are thousands of WordPress themes and plugins available to choose from, depending on your specific needs and preferences.