again, you're too broad, dude. how to create music from scratch? what kind of music? what length? using fl's included stuff, or plugins, or what? you need to know what you want before you can find it - first rule of research.
take a song and make a mix - i'm assuming you mean rearranging it for remixing. do you know any theory? can you read music? that's your basic idea, there - listen to the song you want to arrange, decide on what you want to do to it (slow it down, change the meter or key, add instruments and harmonies...), and then do it. same thing - you need to know what you want before you can just dive in.
how to use the program to its fullest? keep trying, dude =)