I went for the first time 2 years ago, the best tip I would give you is to read several of the guidebooks. I read 3 before I went and they were very helpful. Also, know which park you want to visit first. Have a plan in mind.
All of the Disney boards are full of tips as well.
The guidebooks I read were: The Unofficial Guide to WDW, WDW with Kids, and WDW for Kids by Kids.
I had a 4 and 9 year old with me.