Lack of responsiveness on mobile screen

I have just launched the website (albeit with a temp domain) but am very disappointed to find that while the site looks great on both computer and tablet screens, it looks terrible on the phone.

I assumed the elements would stack, rather than try to cram themselves side by side. I also assumed elements such as headings would automatically be smaller so the words would fit.

Am I missing a setting somewhere? I admit to not being very technical and I know nothing about CSS, so I’m hoping that I can solve this issue with a few setting changes. I attach screen shots where you can see the issues.

Thanks…I would really appreciate some help with this.
Cheers, Marlene