Usage and difference in meaning.
As I know they both mean to be careful. For example :
Both mean be careful about car (~while crossing the road)
From this question 注意する means to warn.
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Just as in English, there are many ways to say the same thing.
And in most cases, you could assume they mean the same.
気をつける is more like an expression and can usually just be used as such.
注意 is more flexible and used in other ways.
車に注意する -> Be careful of the cars
車に気をつける -> Pay attention to the cars