As a new piano learner, what should I learn first (I don't have any musical knowledge, except I can read sheet music but not good)?It would be great if you wrote consecutively or any book that takes from zero.
It depends on what your goals are, but if you are mainly interested in playing from lead sheets I would recommend "How to Play the Piano Despite Years of Lessons" by Cannel and Marx which will give you enough theory and practice techniques to start playing from fakebooks.
Definitely watch YouTube videos and if you find a video you like, subscribe to that artist's channel. But in addition there are free piano instruction sites online to check out.
I would recommend doing some of these lessons to teach yourself basic music theory. I would start with learning note rhythms and how to read music notation.
Here is a good YouTube channel about music theory without making it to boring and this one is probably my favorite out of all. This is what I did when I was first learning music notation, these flash cards will teach you the basics.
Good Luck, and remember that Practice makes Perfect!