I'd say if starting off, don't start off writing. Learn to write stuff in your head.
It makes you more flexible and spontaneous and is the key for a lot of freestylers.
Jay Z supposedly writes all of his rhymes in his head rather than using paper. And if it is in your head, you got an easier time memorizing it and performing it rather than stuttering on a forgotten word or phrasing.
I'd then find someone else who raps and is willing to work with you and maybe write some songs together. You will learn a lot faster if somebody else is learning also and both of yall work together as a team. I'd say find somebody that is at least about at your skill level or slightly better. Somebody that is too skilled might be hard to work with if you are starting off but don't be afraid to ask for advice.
Unlike singing, Rap is not something that is learned in school (although many rappers borrow a lot of song techniques singers use). It is learned through you own experiences and interactions with other people and other music.
Once you get comfortable freestyling (basic freestyling), start getting into writing and structure. Learn the basic 16 bar verse and 8 bar hook. Study complete song structures verse structures, hook structures, intros, outros, bridges, refrains etc.
Study your idol rappers. Learn from them.
But in rap the main objective is to relate to your audience and entertain them. Sometimes the goal is to offend some but even these songs are made to entertain someone.
Rap is fairly simple for the most part but like piano or any other instrument it can go to infinity. People like Twista, Jay Z, Busta Rhymes, Eminem have fairly complex deliveries but all of them started from the basics.
Once you get comfortable