I would like to bind F1 to play/pause and F2 to next song globally how to do it?
When you go to System → Preferences → keyboard Shortcuts, you should see predefined shortcuts for Play, pause and so on. Click on the shortcut, on the right, to edit them.
However, using F1, F2 and so on may be problematic. Some legacy applications may open up their help whenever you press F1 for instance.