You could start with the flutter documentation. Starting any new language the documentation for that language will have all the important information you need to start your learning.
I see you tagged android, In the documentation it has flutter for android developers that helps you convert your understanding of android to flutter.
You could also find sample projects you could download and go over the code in your own time and look at the documentation at parts you don’t understand that help me quickly understand flutter. You can find small sample projects anywhere for GitHub to YouTube to blogs.