Hi Lara -
The first trip to Walt Disney World is so exciting, but it can be a little overwhelming. There are so many things to see and do, it's tempting to try to fit in everything. Speaking from experience, you won't be able to do it all, and if you try, you'll be exhausted and need a vacation from your vacation. So, where to begin...
I don't know if you've chosen a resort, but that should be your first priority. If you can swing it, I would recommend staying in a deluxe resort that has either monorail access or is located on the Boardwalk. With a 5 month old this comes in really handy for hopping back to your resort for nap times. Don't panic if it's not in your budget, buses are available to take you back to the resort continuously throughout the day.
Next, begin looking through the
dining options and find a few restaurants that you think your family would enjoy. You can begin booking reservations 90 days before your vacation by calling (407)939-3463. This always helps me plan an itinerary. For example, if you plan on eating at Crystal Palace in the Magic Kingdom, then it makes sense to plan that day around activities in that park.
Now, I also have children who have a wide variety of ages and interests. I let each child choose their top 3 "must see" attractions and 3 "really want to do" attractions. We then plan our days accordingly. When my older boys know that their two year old sister really loves Winnie the Pooh, they are much more likely to get excited about riding The Many Adventures of Winnie the Pooh just to see her reaction.
Good luck getting started, and feel free to ask more questions as you plan.