Hop on the bus going to your destination. Everything is well marked to where the bus goes. But they don't go on a circiut for the most part, point to point only.
Nope, just get on. Theoretically, only resort guests or day guests with multi-day passes may use the bus service, but no-one will ask or care, unless it is Christmas Day or the 4th of July, and even then probably not.
Non-resort guests who have Park Hopper/Water Park Fun & More tickets can use the buses/monorails/boats to go to multiple parks/resorts within each individual day at WDW.
If you are a non-resort guest and drive to WDW, don't plan on parking at DTD and grabbing a bus to a park to save the daily parking fee. DTD buses do not run to the parks, just to the resorts.