How do I show posts only from selected category(ies)

I’m trying to set up a small site using WP, but I can’t figure something out.

I posted a test post in a category called “Updates” and then I posted another in a category called “Misc. Videos” and I made sure that the posts were set to only appear in those categories.

But now they’re both showing up on the main page… I don’t know how to change it. I don’t want every post I make in every category to end up on the main page.

I’m really confused… I’ve never used anything like this before, I’ve always just done it myself, but I was told that using WP would be much simpler.

Is there a way I can set the “Update” category as the main ‘blog’ page on my site?