Explain why this statement is incorrect: "if i find something on the internet, that means it is for the public use, and i can do anything i want with it"

This is Copy Right and it means if you change it enough then you can use it.
For example if you were to use an article on penguins in a video as long as it was not the whole video and you had commentary behind said article that would be fine. 

This would be copyright unless you change it significantly.If you don’t change anything you might be fined.

I’m not sure but i need to know too

Which approach what do you mean