You should include your need for a ground floor room in your requests when you make your reservation. If your reservation is already made, just call and ask that to be a request. Be sure to specify that it is needed for medical reasons. Those requests are filled first before general requests would be.
Your wheelchair will be transported by the airline at no additional charge. You can keep it with you in the airport until the plane loads, then take it to the gate and fold it securely. It will be picked up by the baggage people at the gate and brought back to the gate when the plane lands at your destination. This is called "Gate Checking" and you should ask about it when you check in for your flight. The agent may tell you to check with the gate agent about gate checking. You should do this as soon as you see an agent at the gate; don't wait until boarding begins. The agent has to make a special tag for the wheelchair so that it is handled correctly. If you wait until loading has begun, the agent is busier and there is more chance of mistake.
Some planes also have a closet that may be large enough for a folded wheelchair. If you would like to do this, ask ahead of time and again when you check in. The closet is first come, first served and they may only have room for one wheelchair, so if someone else asked before you, you will be out of luck.
You can bring a wheelchair on the DME and the regular resort buses. THere is information in the disABILITIES FAQs thread about the resort buses.
For the DME bus, you can ask for a wheelchair accessible bus with a lift and ride up on that. If you don't, those buses have a rather steep set of stairs.
Here are some pictures.
DME Bus lift in action at its highest level:
DME Bus stairs: