When you connect your domain with your Shopify store you don't need to do anything additional for setup SSL.
Shopify by default setup SSL for your store.
Check here what Shopify say about SSL: https://help.shopify.com/en/manual/online-store/os/domains/managing-domains/ssl
but my problem still occurs some times, when I type my domain on my browser like this https://demostore.com it says "Your connection is not private" photo below is just a sample of what I'm getting on my store.
SSL (Secure Sockets Layer) is a wonderful tool that provides security over communications on the Internet. It plays an inevitable role in the businesses of merchants on Shopify. In fact, there are some reasons that may make the SSL unavailable on your Shopify stores. I recommend you to read this guide to learn what they are and how to fix the SSL unavailable issue.
Hope this helps!