I don't know how much help I can be, faith, but I'll try.
A good place to start is with what you have already figured out. Where exactly in France, Italy and Greece do you want to go? Honestly, you could spend a month easily in those three countries without worrying about anyplace else. If you haven't already, check out the respective guidebooks for those countries. You can probably find several at your local library, or check out the websites of lonely planet
or lets go
for more information. I like to get the most up-to-date guidebook that I can without sacrificing good information-- I'll pick a slightly older Lonely Planet vs. a newer unknown (or known-to-be-crappy) book.
Personally, I think that spending the time in France, Italy and Greece alone would be plenty for a month. But if you want to go to other countries, think about what interests you more -- ancient/medieval architecture, hot sunny weather & beaches, mountains & castles, etc. etc. Once you get that figured out it's easy to start narrowing it down.
Unless you're like me and you want to see it all, in which case it might be a good idea to try to limit yourself to destinations within reasonable distance of where you already *know* you're going to be. Does that make sense???