Thanks for posting your question regarding adding PayPal. In order to provide a more accurate answer, we would need to know what software you used to build your website.
However, these articles related to PayPal may assist you:
PayPal for Woocommerce (WordPress)
PayPal for Prestashop
PayPal Buy Now for HTML websites
PayPal for VirtueMart (Joomla!)
I hope this helps!