We use yet in the question and in a negative sentence and it is always at the end of the sentence.
Examples
Have you had dinner yet?I haven't spoken to her yet.Has he got up yet?
We use just when something happened a short time ago. Just, like already, goes directly before the
main verb.
in positive sentinces
Examples
I have just eaten.She's just found a new job.
We've just finished cooking.