に気をつける【にきをつける】

に気をつける means "to be careful of", "to watch out for", "to look after". The thing you watch out for goes before it as a noun with に.

The noun takes に, never を. The を is already inside the expression, on き気.

• × 車を気をつけてください → 車に気をつけてください

Being careful to do something

To say what you take care to do, or not to do, put a verb in front with ように. The verb is in the dictionary form or the ない-form.

気をつける or 気がつく

きがつく気がつく with が means "to notice": something comes to your attention. 気をつける with を means you direct your attention on purpose.

気をつけて on its own

気をつけて with no object is what you say to someone leaving: "take care", "go safely". きをつけてください気をつけてください is the polite version, and おきをつけて気をつけて is politer still.