To create a shopping cart you'll need to use a server-side language like PHP, NodeJS, Python, Perl, etc. The easiest way is using a 3rd party API or you can do everything from scratch with a library. My personal fav is nodejs and bitcore, but this is usually too much for the average user.
If you're looking for something much easier I suggest Mycelium Gear. They basically allow you to fill out a form with your product info and they'll give you a link to post on your site.
Also if you're a wordpress developer I have heard good things about Cryptowoo in conjunction with Woocommerce. I have not tried it myself though.