How to connect my new light fixture that has 2 (+ground) wires to the ceiling box that has many?
While most people can make educated guesses about what the coloring means on electrical wires, I would suggest using a multimeter to actually test the wires and through a process of elimination, determine your live wire, the return, and ground wires.  Too many ceiling boxes are wired oddly to have confidence in guesses based on wire color.

The live wire will have a proper voltage reading (in AC) to the other two bundles when the switch is on.  The return wire will have a proper voltage reading (in AC) to the live wire, when the switch is on, and will have no voltage reading (in AC) when the switch is off.  The ground wires should have a proper voltage read to the positive regardless of the switch position.

If you can't identify which wires in your outlet using the above rules, odds are something is really off with your wiring, and you probably should call in a professional.

Keep in mind that testing live wires can be very dangerous, so you need to take proper precautions.  If you don't know what those are, call a professional.