PayPal is a very popular system for accepting online payments. It allows a user to accept payment from other PayPal accounts, and from credit cards. It's very easy to set up.
Having a PayPal payment option on your website is almost essential, as many people will only want to pay by PayPal.
That's it. Now people can buy things from your website. You'll receive an email whenever someone makes a purchase. Then you can send the item the visitor has bought.