it depends on exactly what you are looking for here - do you have anything on your forum that you want them to see at all? if not then like @MikeNolan said, you can use the login_required setting to restrict all content to members. but if there are some content you want anonymous users to be able to see, then you have to use other methods, like category security permissions for example.
I do not think there is currently a way to make a specific topic visible to those logged in while other topics in the same category are visible without logging in, but you can do it at the category or site levels…
One possibility might be to move topics you want behind the login to a different category, one that requires at least TL0 to read.
There is quite a bit about how to be a discourse admin in the documentation category here, for more on categories I’d suggest starting with Create a category in Discourse
In all honesty, I think the best way to learn your way around the admin features is to experiment with them, but then I’m a programmer/analyst/engineer by training so that’s kind of what I’ve been doing for close to 50 years.