First of all WELCOME!
There is nothing like a Disney Vacation and a family reunion just makes it better.
As for starting places, my advice would be in this order:
1. Decide when you want to go
2. Decide where you wan to stay.
I would look through these boards (theme parks, resorts, etc) to see others questions and post some of your own. Definitely get a planning book from your library or book store. I personally like the unofficial guide but there are many.
With a large crowd it may be hard to find a time that works for everyone. I am trying to plan one for my extended family. There will be 14 of us (4 families) and everyone wanted to go a different time of year.
Once you get what time of year you want, then you can start looking at resorts and seeing where everyone would like to stay.
Good luck and enjoy it. Planning is half the fun!