Welcome aboard and don't hesitate to ask anything.
Can't tell whether you are staying on or off site. If off-site you should consider a car a necessity. Moreover, even if staying on site, you should consider a car necessity if you intend to go off-site for anything, like going to Sea World or Universal. If staying at a WDW resort and intending not to leave the site, then car is optional but be aware of the following in making the decision:
1. Generally Disney transportation from your resort to a theme park and back, whether your resort uses a bus, monorail or boat, is good. In fact, to the Magic Kingdom it is really the best way to go. To the other three theme parks it may be faster to drive depending on which resort you are staying (from some resorts you can walk to a park) but usally not that much faster.
2. Going to waterparks or Downtown Disney from your resort by Disney Transport is somewhat OK but you can have additional stops and a car is generally significantly faster.
3. If you are planning to go from one resort to another resort then seriously consider a car. That trip often requires taking one bus to a park or Downtown Disney and then taking another from there. That double bus trip can take an hour or more while you could easily drive the same distance in 10 minutes or less. Example, suppose you were going from Port Orleans to Fort Wilderness. The need to take two busses can make the trip take an hour. You can drive it in about 3 minutes because FW is about a mile away from Port Orleans (note you are not allowed to walk outside the resort areas as most of it is very busy streets with no sidewalks).
For any resort they provide you transportation info when you arrive.