This question has been asked and answered time and time again on eNotes. Here is a comprehensive link for you: http://www.enotes.com/john-donne/q-and-a/tags/metaphysical+poetry
It seems to me that you need to write an essay in response to this prompt. While I can't write your paper for you, I can give you some information that should help to get you started.
Metaphysical poetry is poetry that is generally concerned with philosophical and/or intellectual issues. It is characterized by the use of poetic devices such as conceits (long comparisons that show connections between objects or ideas that are not typically or logically compared to one another) and paradoxes (descriptions that at first appear to be contradictory but that seem logical after a closer examination).
John Donne was a prolific poet whose works are usually assigned to one of two periods in his life: he wrote mostly about love when he was a young man, and as he aged (and after he became a member of the clergy) his focus centered more on religious issues.
In order to answer the second question you ask ("How is Donne a metaphysical poet), you should review the poems you've read--or poems you've been assigned to read--for content and form and see if you can find evidence of the characteristics I mentioned above. I hope this is helpful!