So, what happens with the time you spend getting ready, such as paperwork on the ground, doing the preflight, etc? Do you get any $$$ for that? Or, do you start getting paid once they push back, no matter how early you got there to get ready for the flight?

The routine pre- and post- flight activities are are provided by you as a complimentary service to the traveling public, airline stockholders, and managers.

Actually your hourly pay rate has traditionally been assumed to cover routine pre/post flight duties

A few airlines have a provision to pay you for time spent doing non-routine pre / post flight duties. In this case you would get paid for time spent dealing with lengthy Mx issues, passenger issues, or ground repo of airplanes. At most regionals, you just take this stuff in the shorts.