How to add PayPal to your website depends on what software you are using for your website. Some of the most popular platforms for building a website are:
If you are using wordpress, there are some very easy to use plugins available on http://wordpress.org.
Here is a youtube video to add PayPal to your wordpress website using Simple PayPal Shopping Cart plugin (http://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/)
If you are using Joomla as the platform for your website, then there are a lot of Joomla extensions available in the Joomla Extension Directory (http://extensions.joomla.org/) to easily build an online store.
Virtuemart is one of the free extensions that can be used to create an online store using Joomla. Here is a brief video on how to install and configure Virtuemart on a Joomla website.
The easiest way to build an online store using Drupal is by installing the Drupal Commerce Kickstart distribution. Here is a video on how to install the Commerce Kickstart distribution.