Here's a question I'm asked relatively frequently at cons and other gatherings of fledgling writers: "Is writing short stories first a good way to start 'breaking into' writing novels?"
It's a question to which I honestly don't have a firm answer. Many of the writers of my generation (I'm old, after all...) 'broke into' the field by first writing and publishing a body of short fiction before moving on to the novel market. For many of us, that strategy worked -- at least I can point to several people who are still regularly publishing novels (and short fiction) who followed that route -- so the strategy worked for them. For me personally, I found that the 'short' fiction I was writing just naturally became longer and more complex, that the tales I wanted to tell needed more and more room and it was obvious that I needed to learn how to write novels. But...
I do have some doubts as to whether deliberately starting with short fiction is a good strategy or not. As with every other strategy to learn an art, there are advantages and disadvantages. Here's how I see them...
The advantages should be fairly obvious: short fiction allows you to learn the basics of the craft of fiction. The added advantage, though, is that writing short fiction alllows you to complete works far more rapidly. This means you can quickly experiment with voice and structure, with different genres, with different stylistic devices -- and if the experiment fails (as many will), well, you've 'lost' a few weeks instead of the nine month to two years it took you to write that deformed and hopeless lump of a novel. With short fiction, you can find your own individual voice before you try writing something as time-consuming and monumental as a novel.
(An aside here: You never really 'lose' anything from a failed experiment, not if you have the right mindset; there is a great deal of knowledge in failure...)
Other advantages: publish a bunch of short fiction, and you will have a built-in audience for your first novel; publish a bunch of short fiction, and you'll have a nice set of credentials to lay out in front of that agent or editor when you're shopping your book -- heck, you may even have an award or two to toss in. I can guarantee you that having professionally published a body of short fiction will help you when you start looking for an agent or publisher. I know this because my first agent told me that one reason she agreed to look at that first novel manuscript of mine was because the short fiction sales demonstrated to her that I had a certain level of competence as a writer. Without those sales, she might not have asked to see the manuscript.
This is all good. Yet in many ways, short fiction and novels are rather different beasts. And that leads us to the disadvantages...
I would say the skills you learn in short fiction don't necessarily translate into equal skills for writing long fiction. I teach Creative Writing, and there's a reason that I have a separate Novel Writing and Short Fiction courses: because short fiction and long fiction require different skills and approaches.
The pacing is different: a short story needs to start as close to the end as possible while a novel may start much further back from the climax. Structure is different: the way you build a novel is often not something that you can duplicate in short fiction, as novels use a more intricate structure (and on the flip side, short stories can often use wildly experimental methods that work within the confinement of a short story, but which would get deadly tiresome to the reader in a novel). Plotting is different: short fiction tends to have a 'straight-line' plot; a novel's plot is generally more complex, and has the added complexity of sub-plots supporting the main plot.
Scope is different, since short stories tend to use a microscope while a novel uses a wide-angle lens: you can tell the tale of a battle in short fiction, but ony a novel can give us the whole five-year long war. Setting is different: you generally have one or two settings in short fiction; in a novel you might have dozens -- which means that the worldbuilding has to be much more in depth; you won't get away with a painted backdrop in a novel.
Characterization is even different: the character arc in short fiction will usually show the 'top' of the arc -- that defining moment when the protagonist's life is changed -- while in a novel, the writer can show much more of the arc. Characterization is generally slower and deeper in a novel.
Look at it this way: at the core, you can't learn to play piano by learning how to play guitar. Yes, they're both musical instruments and in learning one you will understand some fundamentals about the craft of music that you can take with you to the other instrument. But if you want to really learn to play piano, well, you need to sit down at the keyboard. Ultimately, if you want to write novels, you will have to write a novel.
So I give the question back to you out there: "Is writing short stories first a good way to start 'breaking into' writing novels?" For me, the answer is a resounding, unqualified "Maybe." I would say that if you really dislike reading and writing short fiction, then don't write short fiction -- go and write your novel instead (but expect that you probably won't sell the first, or second, or third or maybe fourth or maybe even fifth novel, just as new writers rarely sell the first few short stories they produce). But if your passion is novels, then go with passion.
On the other hand, if you enjoy reading and writing both short and long fiction, and you want to learn the craft of writing and the basics of marketing as quickly as possible, well, then starting out by writing short fiction might well be your best strategy. You'll still need to understand and learn the differences between the two forms, but you'll more rapidly master the commonalities they share, and you'll have 10 - 20 pieces of your work out there circulating each year, not just one or two -- and hopefully a few sales under your belt when you do try to sell that novel.