Responsive Website - using html, css and some bootstrap only

I have a lot of trouble making the website responsive. I have created another website while following the instructions of the course instructor, and it was a lot easier than doing things myself.

Eventually, the website I was able to build (and I took the simplest challenge here) is not really responsive...

Would love to have some comments or ideas how to improve the way I code to make this website (and my next ones) responsive without spending hours on it :)

Thanks!