Poetry, short stories, novels, creative non-fiction, drama, memoirs--all the various genres of creative writing have their own protocols, not just in writing but in submitting and publishing as well.
a. Poetry. The market for poetry has always been difficult, and it is difficult now. And there's almost no money to be made. This may sound like the start of a cruel joke, but is nevertheless a reasonable observation about poetry's relationship to the field. People do not usually get rich writing poetry. Still, it may be the most exalted of the genres, both for historical reasons and for the very fact that its foundation is its own sense of art, its own sense of itself, and is not a practical means to an end. People write poetry because they actually want to write poetry. But try explaining that to your parents.
b. Short fiction. The market for short fiction has often fluctuated, as short stories go in and out of market popularity. Of all the genres, the professional wisdom regarding what to do with a collection of short stories is the most questionable, given the apparent volatility of the genre. Still, short stories seem to survive regardless of all the doomsaying.
c. Novel. This is the standard in the publishing field--that is, and crassly said, this is where the money is made. Novels also take a substantial amount of time to write, and therefore take a serious, professional commitment. Publishers are most interested in novels.
d. Memoir. A first book will rarely be memoir, unless there is some special circumstance, or if the author is writing this book at age 70 after a long and rich life. Memoirs have to offer something new, even as they are based in something already known. Personal perspective is everything here--that is to say, interesting personal perspective is everything here.
e. Drama. Drama has its own rules and its own world, quite apart from the others genres of literary writing. Even the look on the page of a play distinguishes it from other forms. Quite literally, the words on its pages are meant to be spoken, even as they are being read. As a result, the market is understandably different for plays. The point is not necessarily to get them published--which means they will be read--but to get them produced, where they will be spoken, or performed.
f. Screenplays. As with drama, this field has its own rules and look as well, and these protocols are even more absolute. This is a cutthroat business, as there is indeed potentially a great deal of money to be made. Submission of screenplays, for example, is in 10 pt. Courier, and there are length limits attached to various forms within the genre, such as television sitcoms. If you don't know this, producers will simply move on to someone who does. The writer in "Hollywood" is not the center of the universe. Of all the genres, this is the most visual. Everything in a screenplay must be filmable in some fashion. Also think about hiring an attorney in this area.
f. Creative non-fiction. This is currently a hot field, for what could be any number of reasons. In their best guise, creative non-fiction pieces are kitchen table conversations, an art form that is as old as old itself. This art form talks about things, and doesn't have to do much else except be good listening.
g. Essays. This genre can often be described as creative non-fiction, though it may be characterized by a more elevated tone and foundation, likely expressed in a relatively didactic fashion. Essays are most often found in learning environments, such as schools and libraries. A book of essays is not often a best seller, but some of our longest selling literature is in the form of essays.
h. Work for hire. This genre would include commissioned articles, ghost-writing, business manuals, specific translations, reports, and so on.
DVICE. This of course is too short a space to explain the various intricacies of the genres. What is important to understand is that each one clearly has its own rules, and learning them is a prerequisite for success. If you are uncertain, or don't know any of the protocols, reading any reasonable book addressing the field will do--and there are many of them--but find one. Don't guess. The devil, as they say, is in the details.