which keyboard keys to be set for those 16 keys which can easily press together.
There are no "magic keys" which you can use on any keyboard without problems.
To do it correctly, I recommend reading my article at
http://www.mameworld.net/emuadvice/keyhack2.html Scroll down to where I say "How to do it, Section A, using any type of keyboard".
Or if you are lazy and in a hurry, run one of the programs under 6. Verification (Keyjammin or Keyscan), have a friend help you mash 16 keys and whatever they are are the ones you can use.
You will then have to map these keys to figure out how to hook up your buttons, so you're not saving that much effort.
NOTE: Sixteen keys will get you a panel with 4 buttons per player, but a lot of fighters want six. I explain how to do this with a 16 by X keyboard matrix and allowing the opposite joystick directions to block, but you HAVE to map out the whole matrix to use this method.