Done should not be used alone as the verb of a sentence. It is a past participle.
It must always follow an auxiliary verb like has, have, am, is, are, was, were, or one of their contractions.
Incorrect: I done what I was supposed to do.
Correct: I have done what I was supposed to do.
I've become aware that some English teachers take offense at the use of "done" to replace "finished," and who correct their students who say "I'm done." I'm not one of those; I suppose nitpicky people could fuss and insist that people should say they're "finished" doing something.
Correct (with contraction): I've done what I was supposed to do.