What are the most important ideas of Immanuel Kant?

As little as possible. He's virtually incomprehensible

You mean you’ve tried to find out yourself, but just ’Kant’?

Fine, fine. Crib-notes version then:

1) Ethics: Kant’s “Categorical Imperative” (always and only live by those rules which every person could live by)

2) From C.I.: treat all people as “ends” unto themselves, never as “means” to some other objective. Hence, you may never lie. EVER.

3) Logic: the analytic/synthetic distinction (later argued into arguable irrelevance by W.V.O. Quine. The notion being that analytic statements aren’t really news, because they can be figured out without conveying further information about the world- such as that a triangle has three sides. By contrast, a synthetic statement is 'new', adding information. It couldn’t have been known otherwise.

4) Logic: Kant’s “Transcendental Deduction”, which is a method of claiming that certain facts must be true by virtue of the way the world is. This method grounds his Categories, as well as attempts to refute David Hume’s contention that there is no reason to believe in causes of events.

You probably want to check out his wikipedia page, at least.