Music is crucial to your project. It's the secret ingredient that goes directly to a viewer's heart.
I've been wrestling with music on my sci-fi feature, In-World War, which is currently in post-production.
It took me a while to figure out a process that works for me, in deciding which tracks go best where in the film, and which parts of the movie to score (i.e. compose new music specifically for a given scene).
If you want to learn from our pain and misery (highly recommended instead of going through it all yourself), find out how we are facing the challenges of selecting music for our film.