For music compositions(especially songwriting), there are multiple different approaches, from story to following by ear. Follow the notes with harmony or not depending on how you want it to sound. You should try reading about music theory a little to understand this context further though.
I also have started to collaborate with some other fellow sound designers to write a comprehensive guide on sound design. We’re probably going to publish it on the forum as a tutorial.
It is as if you ask: how can I build a house, without knowing anything about engineering, architecture, mathematics, etc.?
In this case, it depends on what you want: if you want something fast and well done, hire a specialist.
However, if you want to do it for yourself, you need to learn first. So, we are talking about a dedication, at least, a few months of study to start getting some notion.
And currently, you will find endless tutorials on the internet for that.
Sorry for the frankness, but you don’t make music like you make a cake …