They pick up at the bus stops around the resort every 10-20 min.s. I think they start about an hour before park openings, or even earlier. There's a separate chat forum uunder "The DIS Discussion Forums" about transportation that might help you more.
there are early morning breakfast busses. So, if you want to make an ADR for Chef Mickey's (for example) at 7AM, there will be a bus to take you there.
The rule is to be at the bus stop 45 minutes before your ADR (yes, the busses will be running before 6:30 for the MK area ADRs). A bus will show up.