Struggling with learning testing - eviltoast

I am really struggling to include proper testing practices into my code and would appreciate any advice on how to get going. I work in web dev so my I am interested in how to properly implement a suite of tests for websites and incorporate into it a CI/CD pipeline.

I find a lot of tutorials teach the most basic types of unit tests, 90% of the time most instructors teach how to write a test to sum two numbers, but when it comes to writing real unit test I find it hard to know what I should be testing. I learnt some cypress and have gotten better at including end-to-end testing because that makes more sense to me, but I still feel I am way short of where I should be.

How can I move forward? Did anyone else find themselves in my situation and find good resources to help them learn? Thx