Portfolio Page Basic Help

Hi guys! So I just started my portfolio page and I thought I had a pretty good grasp on what to do, but I’m honestly struggling with even basic things haha.
So my first thing is, how do people just have background images and then add text and images on top of them? I always struggle with the sizing and just trying to put things on top of background images.

This may help you get started:


With sizing you might like to read about and try Bootstrap, or research css terms such as box model or flexbox