First off, let me stress that this is nothing more than answer, not necessarily the answer. There is no perfect way to get into comic writing, to get that first job. It's not like other careers where you can take certain courses, earn a degree, and head for a job fair or look in the classifieds. Comic book companies are NEVER looking for new writers, but are ALWAYS looking for new writers. This is without a doubt the question I get asked the most (maybe tied with "What was it like to work with Michael Turner?" Short answer - Awesome!).
So, this is only my response. My own personal opinion on how I would go about things if I were trying to break into comics today. Let's start with the harsh truth. Nobody is going to look at unpublished material. Sorry, but that's a fact. You got that epic 12-part Batman arc already scripted? Too bad. Got a great idea for Spider-Man? Forget it. Penned the first 25 issues of your legendary opus that has yet to be produced? Save it. While editors and companies are all too happy to thumb through sample portfolios for artists, the road is much narrower for writers. All they want is published work. No scripts. No samples. Only finished, published work.
So, that's what you need to do. Create an idea and see it through to fruition. Most likely this means a printed comic. Web comics have certainly jumped in profile and awareness and more people are willing to look at such endeavors, but most still want that printed, published work. It's your calling card in the business, albeit a potentially expensive one, considering the time and effort and funds necessary to produce a comic book. But, there it is. That's your play here. At least, that's what I would do.
Now, there are exceptions to every rule. For example, Kyle Higgins actually got noticed for a short film he wrote and directed that caught Marvel's eye. It’s called THE LEAGUE, and is actually available on iTunes if you want to check it out. But I digress.
To catch an editor's eye, you need to be able to give them something and that means a comic book. Create your own property and develop a comic book for it. Don't worry about having the entire 100 issue run mapped out. Focus on the best idea you got swirling around in your head. The idea that will work the best as a comic book and the one that will let you showcase your talents the most. You will be pouring a lot…and I mean A LOT of blood, sweat, and tears (actual physical tears) into this thing, so make sure it's an idea you believe in. Think it through.
Got it? Good.
Now you can overcome the next hurdle. An Artist. If you have the funding, go nuts. Find the best artists that money can buy - illustrators, colorists, letterers - the whole gang. But if you are like most of us, life is the very definition of a "fixed income." In that case, find an artist in the exact same boat as you. Someone hungry to get noticed, to get into the business, but hasn’t been able to crack that nut. You can find them in a host of areas. Your local comic book shop. Art departments in nearby colleges. Artistic online communities like Deviant Art. Regional and local comic conventions. It's almost like putting together a garage band, only you're interested in making kickass comics instead of music.
Basically, look for the best artist you can find, whose style matches your idea, and one that has the same drive and work ethic that you have. Pay attention to that second one. I have heard many a story of writers who partnered up with aspiring artists only to wait eight months for 2 pages of art. Don't get me wrong - good art takes time and you'll most likely be working with someone who has a day job like you. Someone with passion, but also bills to pay, because let's face it - you sure as hell can’t afford to pay anything for the pages. Essentially, you go into the project 50/50. You write it for free; they draw it for free. And, you both hope to reap the benefits down the line.
In a perfect world, under the best of circumstances, your project becomes the next little engine that could, defying the odds and making it into stores across the country and turning into the next Walking Dead. And, I hope that is the case, especially if it's good book because I love nothing more than great comics to read. At the very least, your effort will produce a viable, published comic that can get into the hands of editors and publishers in order to further your career. Obviously, there is a lot that goes into publishing (and distributing) your comic book, but that would be a much longer discussion.
The bottom line is that reaching this goal, publishing your own book will put you leaps and bounds ahead of so many people who "want to write," but don’t actually get around to doing any actual writing. Not only will you have a physical manifestation of your work to share and sell to people, but the published work will show potential editors that you have the drive and determination to see things through. The writing in your work reveals talent. The finished book itself reveals tenacity. Both of which are key to breaking into this business.
Again, in my opinion.