How do I keep the context box and sidebar level?

I'm kind of getting back into coding after not touching it since high school, and to practice I'm making a webpage for my开发者_如何转开发self. I'm trying to have a sidebar and content box, however when the content box gets filled with text, the sidebar moves down. How can I keep both boxes stuck to the top? Image here

In the CSS file I tried position: sticky, fixed, relative, etc but none of those did what I needed it to do.