how to IDE multi files

my main c file is getting bigger, how could I split it to multi files and include them in main c file?

http://forum.arduino.cc/index.php/topic,130895.0.html after read this I still don't understand. for example I have a sketch abc, which is in a folder abc/ I want my abc sketch include "a.h" and I want a.h also in folder abc/ I even could not create a.h help me.

got it http://jamesreubenknowles.com/multiple-source-files-with-the-arduino-ide-1618