Before you sit down and start writing, you need to ask why you are writing in the first place. Different people write for different reasons, and some of those reasons don’t make for great writing.
Money: Let’s nail this one right off the bat. The odds of you making any money at this are slim to none. You may make enough to pay for materials, but that’s about it, and that’s if you are extremely lucky. The problem is that most people either don’t go in with a business plan, or they go in it specifically for the money. Because this is a business that depends so heavily on the art, you can’t really predict what kind of money you will make; the art will either be incredible or mediocre, and either a fan favorite or not. And neither has anything to do with the other.
Fame: This is most likely, but still not a good enough reason to start a webcomic. Fame in any art is fleeting; that is exacerbated in an internet-based art-form, given the web’s time dilation effect. If you want fame, try a rock band instead or an acting career instead; at least you will have something permanent to remember your fame by, not to mention some interesting stories.
Advertising: This is sort of debatable. The idea is that the website acts as an advertising panel for your art. The problem with this is that, because you are trying to run two entirely different kinds of business. The art business requires a certain kind of mindset; you have the same kind of self-promotion, but you can go for weeks or months without having to post something. A webcomic, however, requires a posting at least once a week. Also, an art studio can be dependant on just one type of art (photography, painting, what have you); a webcomic requires not only good drawing ability, but good writing as well. Even the best artists don’t have usually decent story-telling skills, and without them the comic falls flat. As it was your source of advertising, your business dies quickly. Seriously debate this.
I Needed A Hobby: NO. Just run away now. Don’t look back, and have no regrets. A webcomic represents a commitment. I know that a lot of hobbyists are capable of some extreme commitments, but this requires a long-term boring commitment; this is a marriage-type commitment. There will be problems, you need equipment and software that isn’t cheap, and this is not good for a momentary rush. If you need a creative outlet, take up painting.
Because You Have To: Now, we’re talking. The more passionate that you are about what you want to do, the more successful it can be. If you need to draw, and you don’t feel good unless you have written or drawn something, then that’s the best sign. You may see it as way to the Major Leagues (Marvel, Dark Horse, Tokyopop), and you need the training. But this is something you see yourself doing for a long time, just like you intend to breath for a long time, but you consider breathing optional; without the strip you are nothing. That’s what I’m talking about, and that’s the only acceptable response.