Reversing motors

I purchased a 3d printer, Prusa i3 kit, and put it together. Testing with Pronterface, the z axis go in reverse of what they should. When I push plus they go minus and when I push minus they go plus.
What iI would like to know is how to edit the, Marlin Firmware to change this. I know you can reverse the wires but I would like to learn how to edit the firmware.
I have never used a programming language., can you suggest a book I can get to learn how to program, for dummies like me?
Also the x axis when I push plus it will plus and other times it will when I push plus, it will go minus. The y axis is ok.

See these settings in your Marlin/Configuration.h file:

#define INVERT_X_DIR true    // for Mendel set to false, for Orca set to true
#define INVERT_Y_DIR false    // for Mendel set to true, for Orca set to false
#define INVERT_Z_DIR true     // for Mendel set to false, for Orca set to true