If you are at a Disney Resort, you take a separate bus for each of the 4 major parks. There's one for MK, EP, DHS, AK/BB, and DTD/TL. The buses for each park are supposed to arrive at your resort in 15-20 minute intervals. If you're going to MK, for example, then MK will be the only place the MK bus goes.
The buses usually start running at least one hour before the parks open. If you have a super-early breakfast somewhere, I would highly recommend you take a taxi. The buses don't go from resort to resort. They do go from park to park. At POR, you can take the boat to DTD. It's much quicker than the bus.
When you arrive at MK, refer to the sign board for the correct bus space number for the POR bus. The Bus Driver will also announce the correct number when you exit the bus at MK. When your bus to MK drops you off at MK, it may pick up returning POR guests, or it may be re-assigned. The Disney bus system is very fluid, and is designed to quickly respond to high demand situations as they arise.
If you are at a park, and you have a question about transportation or anything at all, stop in at Guest Services for assistance. I hope this helped answer your question.