I don't know much about FNF but a good structure is the foundation of your song and must be done well if you want your song to end up good. I've always had trouble with structure so I'm afraid I cannot write a guide for you on how to do song structure, but don't be afraid to copy a part played a section or two ago again, it can help make the song feel less sporadic, and don't be afraid to key change. Make sure the intensity of your song stays dynamic
About mixing and other stuff, I made 3 juicy forums about that worth czeching out
