If your asking that question...then the best answer is to get out the phone book and start calling contractors to give you some quotes.
You will need to determine if the ceiling above is just a ceiling and that the roof is properly supported. If you don't know that yet, then the task is most likely beyond your capabilities. Call in an expert.
Other factors...thickness of roof joists.....spacing......sheathing....span....lots of little details.
Even if you are on the right track, you will still get run over if you just sit there.