Finding your genre...

1. Write the book you want to write.

2. Take a look at the main conflict focus for a hint as to what genre it might fall into.

If the conflict focus is on interpersonal relationship, it's likely Romance or Literary. The distinctions between those two are obvious.

If the conflict focus is on the internal, then it's likely Literary.

If the conflict focus is on the external, take a look at the type of external and what's involved.
Solving a crime? Mystery.
Stopping the baddies from extreme destruction? Thriller.
Discovering/exploring some technological or scientific concept? Science Fiction
Discovering/exploring something utterly non-real, magical or legendary? Fantasy

I think you get the picture of how to figure things out from that.

Personally, I don't really think about it much as I'm starting to write the book. The story is the story and genre is marketing. There is a time and place for marketing and it's not when you're creating the story and getting it down on the page.

As for whether or not to start writing two books at once--try it. See if it works for you. If not, then you know something not to do next time. Figuring out how to write is completely trial and error--always a learning experience.

Good luck to you.

I'm with Deleyan -- the best thing to do is let your writing drive your marketing, not the other way around.

If you're torn between multiple ideas, go with the one that seems the most compelling to you and STAYS the most compelling. Also, notice what you do and don't do well; it makes sense to play to your strengths, not only for marketing reasons (although that is obvious) but also because you'll enjoy the process more.

The stories I want to tell are full of the fantastic, the magical, the occult, the scientific, the futuristic, and so fantasy and horror and science fiction are obviously my genres as a writer. But like you, I'm a reader of pretty much every genre, and all of them contribute to my writing in one way or another.

You're right to note that a basic idea can be adapted for more than one genre. Good old "boy meets girl" can be made into a straight contemporary romance, chick lit (funny contemporary romance with name brands), a historical (Centurion meets girl in Gaul), science fiction (starship pilot meets fem-form android), suspense (boy meets girl in peril or vice versa), "literary" (boy meets girl and spins mental wheels about it), and so forth. And all of the above can be either adult or YA, depending on the ages of the "boys and girls" involved.

You have the adaptable idea. Now choose the characters and setting that most intrigue you, that will most easily hold your attention for the long haul, and go with those. They will determine the genre.

Most fiction will eventually require research. But, yes, you'll likely do much more in the genres of science fiction and historical fiction. Fantasy, too, can call for research, and both it and SF benefit from some preliminary world-planning. Don't let this stop you from writing the work you most want to write, however. You can write and research at the same time, and the research will give you lots of ideas in midstream.

It's not at all uncommon, particularly if you're young and new to writing, to be uncertain of your literary direction.

So write everything. Really. Write everything.

Keep a journal (blog), write poetry, start as many novels as you have ideas for, write shorts and try your hand at some nonfiction. It's important for any writer's development to write much and often. In time, strengths and weaknesses will become apparent. Passion for a particular project will shunt others to the background and you'll realize you've found your niche. (At least for the time being.)

It's a waste of youth to wear only one hat.

I refuse to be pigeonholed. I read pretty much anything. I've written poetry, horror, fantasy, sci fi and mainstream. My current WIP is historical. I suppose whichever novel is first published will determine my genre.

While it is a good idea to familiar with the genre you want to write, you don;t necessarily have to write the genre you read most often--or at least I certainly don't.

I go through periods of writing a lot of different things, then I will pick one out and decide 'this is the one I will finish'.

I devoured Poe, Tolkien, Greek myths and scifi classics quite young so I think they may have had a formative role. The silliness I attribute to Dr Seuss for my parody stuff. No one says what to write, just go with whatever tickles your fancy: read, research, have fun with it.

I recently had to rethink what I wanted to write and change genres. The one I had been writing in, thriller, changed while I was writing my last book. Unfortunately, it changed in a direction I didn't want to write, so I either had the choice of writing a book I probably wouldn't be able to sell or try a different genre.

What I did--thanks to The Marshall Plan for Novel Writing which had a chapter on this--was identify the pros and cons for each genre. What drew me to each genre? And what was I going to find problematic in writing for it? For example, while Young Adult has a lot of action, which was one of the pros, the con was that I didn't want to write about children.

Maybe you could try something similar and see what you come up with.