generally poems come to me at random times like driving down the road, while in the shower and sometimes even while im about to fall asleep. the best thing to do is write it down when you can get to some paper and a pen. then once you have written it down you can type it up here as a hub or in blogs or wherever.
as far as starting a poem its usually something that just comes to mind and not really something to think to hard about. a good poem just flows out of you like water from a faucet.
that's my opinion anyway others migt feel different or have better advice or answers for you