After some courses I wonder if I'm doing good or not, because the quizzes seem far easier than the info in the videos.

As you say, you're new. Stick around and you'll find that the quizzes AND challenges become more difficult as you progress. And sometimes not. Even now I'll come across a quiz I find easy. If they were all painfully challenging I'd think too many students would drop off like flies. Focusing on quizzes? Do you mean retaking them? I never do, I move on unless I really didn't understand some of the question(s). There is a Workshop section in the Library for reinforcing the material you've learned. You don't want to do this immediately. Watch the Workshop video How To Learn to understand why. And read up on Spaced Repetition if you haven't already.

I think quizzes just make sure that we pay attention to the contents of the lectures, but it's important to apply what we learn ASAP. I find it very helpful to build my own website little by little as I learn each day. Hope this helps :)