MomofTwo,
I'd start by getting one (or all) of the guide books. The ones mentioned most here are The Unofficial Guide, Birnbaum's, and Passporter. They have loads of info.
Check out this website along with
www.mousesavers.com and
www.wdwig.com. Mousesavers.com has info on all sorts of discounts for Disney, including resort discounts and ticket discounts. wdwig.com and this site have lots of general info, pictures, menus, etc.
It's a little hard to tell "newbies" what not to miss in only 5 days. We need a little more info to help you. Where are you staying: on-site or off? Will you take advantage of EMH if staying on-site? Do you want suggestions for sit down restaurants? Character meals?
One suggestion I can give you. Definitely see the closing activities at MK (Wishes), MGM (Fantasmic), and Epcot (Illuminations). They are fantastic.
Your children will probably enjoy MK the most. You may want to plan the most time in this park. Definitely schedule a character breakfast. Check the above websites and/or guidebooks for info on the various breakfasts.
If you haven't been in at least 20 years I'm assuming the only parks were MK and maybe Epcot. You must have been young then. With that in mind understand that Disney World is VERY spread out. You cannot walk from park to park, etc. Leave time for traveling from you hotel to the park, or from one park to the next park.
Please ask lots of questions here. People on the DIS are very friendly and helpful and you will learn so much.
Good luck with your planning.
P.S. If you want to visit a water park you should get a 5 day Park Hopper Plus (not just a 5 day Park Hopper). The 5 day PHP gives you 2 plus options which can be used for a water park. Unused park days and plus options never expire.