Creating a subroutine in AppleScript is pretty easy. Jumping right in, here's an example of an AppleScript subroutine that adds one to whatever you pass into it:

-- a super-simple subroutine
on addOne(a)
   return a + 1
end addOne

-- test the subroutine
set answer to addOne(50)
display dialog answer

I create lots of little subroutines to modularize my AppleScript code, and I encourage you to do the same. Every subroutine should do one (and only one) thing, and do it well.