Things to consider are the essential elements of the music business, and understanding how the music business works, and these days that includes also having a grasp of modern technology (even if that is only enough to understand the need, and the ability to outsource).
For example:
Contracts and Negotiation
Marketing and Promotion
Song Plugging
Music Publishing
There are other areas and roles but those would seem fundamental.
So you write songs.... if you want to make a living writing for other people, and specifically working in-house at a label or publisher (those jobs are few and far between), then you have a lot of work to do.
Stating the obvious, if you want anyone else to use your songs, they need to first be aware your songs exist. That prompts several obvious questions:
Who do I need to get my songs in front of?
How do I get my songs in front of them?
What format should the songs be in?
Is there anything I can do to make it more likely they want to use my songs?
Assuming that you do get your songs in front of the right people, you need to:
know what is a typical deal
be familiar with all main components of a deal
gather intelligence on current deals
know how to negotiate
there are more elements and questions besides these. Fundamentally, you need to learn how to turn your high level questions into more precise questions simply by breaking things up according to common sense. Generally asking who, what, where, when and why at every section / step.