newbie: blog at root level?

I’d like for my blog to load when either
https://www.domainname.com
or https://domainname.com
are typed in. Is this possible?? Most people seem to have installs in a subdirectory:
https://www.domainname.com/blog
I’ve tried installing once — it worked, but setting the URL for the blog to the root level didn’t seem to work.
I noticed that my index.html file was still present, so I moved it somewhere else, but still — no luck. I don’t know anything about PHP — wouldn’t apache load the index.php file if index.html is missing??
Thanks for your help. I appreciate it.