Uh... there's a tutorial that stoffe wrote about it here (
http://www.lucasforums.com/showthread.php?t=173587). 
 ^Well, that's on Local Variables, but Local and Global Variables have similar principles, the only difference is that Global Variables are... global :p, while Local Variables exist only within the module that it is set in. 
 Global Variables/Booleans are set with two fairly simple functions 
 int nGlobalNumber=GetGlobalNumber("your global number");
 SetGlobalNumber(nGlobalNumber, "Value")
 //The "value" is a range between -128 and 127. You can use values to keep track of completed tasks in game. 
 int nGlobalBoolean=GetGlobalBoolean("your global boolean")
 SetGlobalBoolean(nGlobalBoolean, TRUE);
 //Booleans always start out FALSE 
 You also should check via 
 if (GetGlobalNumber("your global number") == Value)
 //Value is again, -128 through 127
 //You can also use other inequality symbols instead of "==" like ">" 
 if (GetGlobalBoolean("your global boolean"))
 //This checks if your global boolean returns TRUE 
 There are a whole bunch of great examples in silveredge's Brotherhood of Shadow mod, specifically the first module of the Orion.  
 //sales_person8.nss
 int StartingConditional() {
 int i = GetGlobalNumber("Tar_PazNik");
 if (i == 4){
 return TRUE;
 }
 return FALSE;
 } 
 Hope this helped a bit ;)