If you are distributing books through StreetLib, the system will automatically propose your own books to add to your bookstore. You can choose to add all or some of them.
Otherwise, or on top of it, you can add books by clicking (big surprise) on the button "add books". You'll see a dropdown menu appear, giving you diverse ways to search for books among StreetLib's catalog:
You can also decide to add the entire StreetLib catalog to your bookstore but it may prevent your bookstore to get identity and coherence.
Anyway, you'll always be able to add or remove books from your bookstore even once online.
For instance, to put into my "Bedside table Bookstore", I'd most probably add a bunch of classic novels. So, I'll pick "by category", select "Fiction" and the child category "Fiction/classics". I'll then be able to either add the whole category or select the books I want to add.
At any moment, you can click on "preview store" to see what your store will look like. You'll find this button next to "add books" once you have added at least one book: