Welcome to the DIS!!! You will find a wealth of information here. Keep asking questions and you will get answers.
I echo the advice to get a good guide book. The Unofficial Guide has a wealth of information as well as the Official Birnbaum Guide or the Passporter. Each of these offers something different and they are all helpful in some way.
I too recommend the Dining Plan if you like at least one sit down meal a day. It is $38/person/day and you get a TS meal including an appetizer, entree, desesrt, Tax and Tip, as well as a CS meal and a snack. There is a special forum at the top of the Restaurant Boards here on the DIS that will answer all your questions about the plan. We (3 Adults) saved $450 over the cost of the plan in October. Remember though, a sit down meal will take an average of 1 1/2 hours of your park touring time. You will have to decide what works for you.
Also check out the restaurants and menus at allearsnet.com. This can help you both decide where to eat, and once you have done that it will help you decide whether the Dining Plan will work for you as well as how to plan a meal budget. Food is expensive at Disney. I agree with the recommendation to go to the Hoop Dee Doo Review. It is pure Disney. Down home food (Fried Chicken, Ribs, Corn etc.) and a show with corny jokes and singing etc.
Check out the hours on the Official web site and make a tenative itinerary of which park which day. Are you early risers who will be up and out at park opening? If so the morning EMH will be great for you. You can get lots done in those first hours of the day. If not, you might want to avoid the EMH park on that day as it gets busier than usual as the day goes on. One strategy I have seen is to visit a park the day after it has had an evening EMH - folks say it is less crowded that day.
Once you have gathered some information, have a family meeting. As another poster said, one of the most important pieces of advice for you is to realize that you can not do it all in 5 days. There is so much to see and do at Disney. Have each member of your family come up with a list of "Must Do's" (maybe 3 or 4 each) and plan from there.
Planning for me is also as much fun as the trip itself.
Feel free to come back and ask more questions as you learn more and Have Fun Planning!!!