How to set a background image of an html file using a div?

I apologize for the dumb question, I am very new to HTML/CSS and I find getting specific answers very confusing at times. I'm trying my best.

I wanted to upload a background image, and I was wondering how to do so using a div. I wanted to use a div because I don't know how to change the opacity of the background image without changing the opacity of the entire html document, so i figured using a div would allow me to do that. However, I'm not really sure how to use a div in this situation.