How do I regulate posts on the main and subsequent pages?

I am reasonably knowledgable in html and css, but I am a newbie to WordPress, but I've been reading everything I can and still don't understand some of the "basics."

I understand about posts. How they are created, how they are published, how you can apply categories to them etc.

But, say I have 10 posts in two subjects total. But, I only want posts in the first subject to show up on the main page. How do I do that?

And, if I have five subjects, how do I get the five subjects to show up on separate pages?

TIA