DD's first trip was at 9 months old and DS's first trip was at 8 months old. Those were the easiest trips!!!LOL Now they are 8 and 3 and it is alot harder. We have taken them every year and you will be surprised at how well your baby will do if you take it slow and go at his pace. Here are some tips:
1) Take your own stroller from home. One that folds easily, is lightweight, but that reclines for naps. Your baby will need a stroller at the parks and it's great for the resorts and DtD as well. The rental strollers are not comfy and do not recline. Your baby will be much more comfortable in their own stroller they are familiar with. Make sure it has a sun shade and you could also purchase a fan to attatch to it.
2) Get your baby use to sleeping in a pack n play before you leave. WDW resorts all have pack n plays in the room for babies to sleep in. Some kids aren't use to that and don't do well sleeping in an unfamiliar place. We got the kids use to the pack n plays ahead of time and it worked out great.
3) Take it slow and realize that this trip will be different than ones in the past. You will not be able to see everything. You will not be able to do comando touring unless you want a very unhappy baby! There are plenty of rides and attractions to take the baby on on your lap. Seeing and touring WDW through a babies eyes is priceless. Take time to enjoy the little things.
4) Get a guide book and pick out your must sees before you get there. Have a good plan and know where you will go on which day.
5) Plan for some down time. Kids love the resort pools.
6) Be ready for your plans to change at the drop of a dime! Kids are unpredictable so be prepared to leave the park if you have to.
7) Take advantage of the baby centers at each park. You can feed the baby in there, let him color and watch some toons on the tv just for a break from all the stimulation and crowds. Also it's a great place to cool off, change a diaper, and relax for a few minutes! They have baby food and toiletries for sale, a full kitchen, high chairs, changing tables, restrooms, etc....
8) Bring any favorite toys from home with you. If your baby uses a pacifier, bring extra of his favorite kind!
9) Ship diapers, swim diapers, wipes, and baby food down to your resort ahead of time.
10) Your baby may or may not be walking yet, but you'd be surprised, most kids that age are happy in their strollers just taking in all the sights! The characters also always give babies special attention!
That's all I can think of right now. If I think of other tips I'll post them. You can also do a search on these boards with this subject, it has been talked about alot in the past and there is some really great advice out there.
Good luck and have fun!