I'm not a screenwriter either, but I know the answer: Novelists do all of the above.
Typically, though, they do what they do best, which is write novels. If the novel sells to a publisher, and does fairly well with buyers, eventually it may come to the attention of someone in film production, either naturally or with your agent's help. At that point, the novelist makes the decision about whether s/he can adapt the work for the screen or whether to sell the option without such a clause.
While many of the skills of novelists and screenwriters overlap, being able to do one well doesn't mean you can do the other. If what you want to be is a novelist, rather than a screenwriter, I wouldn't mess with a screenplay at this point. Pick a single goal--selling that novel--and work toward it.
If, on the other hand, you want to be a screenwriter, stop writing fiction and get busy writing screenplays, with the knowledge that your first is likely to be awful. There's a learning curve.
Maryn, who'll stick to novels, mostly