For a first VN, I would recommend keeping the story short and with a few endings/choices as possible. Just to get a feel for the writing.
Here's a very brief look on short story outlining and conflicts.
And a much more in depth series on coming up with a story and how to plan and structure it.
I think once you have an idea of the story outline, it'll be easier to figure out the kind of art you're looking to make.