I'll refer you to the relevant FAQ article:
https://www.zen-cart.com/tutorials/index.php?article=14
I've never used shared SSL myself, and I understand it can be a bit of a thorny issue, but the FAQ should give you the procedure.
Just a note on how ssl works, given your posts. All of this content is installed into and served from the same directory. Imagine that your server is a guy behind a service desk. People come up to him, and these people know both English(http) and Russian(https). Parts of their conversation with the guy can be in English, and parts can be in Russian. They can even talk about something in Russian and then talk about the same thing in English. Well, it's the same way with your content. It's only installed one place, but it can be served to your visitors in either an insecure protocol(http) or a secure protocol(https).
Good luck, I hope the FAQ answers your question.
--Joe