Index…

I am kinda new to WordPress and PHP in general, but I have a question about the main page. I am fine with the general structure of the WordPress site, but the thing that keeps getting me turned around is the index page. I want to edit the index page of my theme, but not remove the “my posts” from being featured on my page. I guess I’m just wondering how I edit the main page without creating a new page and making it my main page. The thing is, I wanted my posts to appear on the main page…I just want to edit how it looks.

Just to Note: I am in the middle of creating a new theme, but I still have not found where I need to edit to just edit the main page and not creating a new page.

Thanks,
Chise