I have no music or instrument background.
I am part of film project and I am the music director.
I need to create theme songs for various different characters and situations.
I am given no money so I cannot afford a proper lesson.
I have been given 4 months as learning period.

It is pretty bad. I was thinking of using some free music software to create the music. I can probably invest some money to buy cheap books but no courses. 

Despite all the limitations, I really want to learn the proper material and make good music so what are basic concepts I need? Is there a specific way to learn or is it the same as learnibg physics and math?