umm.... u really should learn a bit more about object oriented programming in c++ before you tackle Arduino libraries. Arduino libraries are typically implemented as c++ classes. There are good reasons why you would want to put things into classes & u need to evaluate if your need for "organizing code" falls in that land.
Google for "C++ classes tutorial" or try this one Introduction to C++ Classes and Objects
if you are merely looking to keep related function definitions "organized" in different source files, you can always create .cpp containing the function defs & .h file containing the function declarations and #include ".h" in both your .ino sketch & .cpp files. There are some gotchas in this approach also (variable scope & the like) and again you need to read up more.