We won't stay anyplace else now. So, Dave gave you one company, but there are many. Search this site as there's a bunch of companies listed.
Decide what type of floorplan you want, meaning what type of space works best for the way you want to use it. Once you have an idea what type, size and style RV you are thinking of, prepare to pick and book a site at Fort Wilderness. The rental companies will be familiar with what site you will need based on what type camper you have selected. Book your site based on that.
Try and figure out how many meals you'll be making at the campsite opposed to eating out. You'll have to purchase supplies accordingly.
Do you want a golf cart? Some people do just fine without them. We wouldn't stay without a golf cart. Just makes getting around easier for us. Many (all?) of the RV rental companies also rent golf carts and you can get them as a package.
You really can't beat the Fort for usable space just for your use. Many RVs have as much available space inside as a resort room on property, and that includes the deluxe's. Pair that with the usable space you have outside the door and I think it can't be beat. Outside picnic table and grill. They just don't have that at the GF.
Good luck to you, it really isn't too soon to start your searching and planning. Please keep us posted as to your progress!