I'm new to this forum and new to emulation. I'm looking to run a couple emulators on my pc to play old games from sega and NES. I would like to use a controller instead of the mouse and keyboard so can someone direct me to finding the best controller to use?

Will any controller work for emulated games?
Do you have to have a different controller for each emulator?
Can a controller used on emulation games be used on regular pc games?
Can you convert a Xbox 360 controller to be used on the pc?

thank you for taking the time to help me out.