How do I get into online writing?

At the risk of stating the obvious, I have to say that in my own experience, the best way to get into writing online is to write online!
The key to success, though, is to ask not where you can create your new empire, but to ask instead where you can contribute to other, already successful online empires. To wit: blogs.
More than anyone else, it’s bloggers, people who run one or more blogs (me? I have four!) and try to keep to a regular publication schedule, that are the most serious, most prolific writers in the online space.
Why is that important? Because just about every blogger I know is also happy to receive guest submissions, which means that it’s as if you have an almost infinite number of publications to which you can submit your work!
But hold on, you don’t want to just write a single article and email it to every blogger under the sun. That’ll be quite counterproductive and can burn some bridges before you even reach them, let alone cross ’em!
To start out the right way, I suggest that you read various blogs and other online publications in your market segment or area of interest to find those that resonate both in theme and voice, then email the editors or owners asking if you can contribute an article of your own.
In that email, I’d encourage you to mention that your article will be exclusive to their site, not submitted or published anywhere else, and that you’d be aiming for a minimum of 200-250 words (depending on the site and its style).
Exclusivity and target length are the two things I always want to know if someone sends me a note inquiring about contributing an article for one of my weblogs.
Before you send any articles in, however, I also suggest that you create a destination site of some sort, whether it’s a Facebook profile page or your own blog or rudimentary Web site. This way, you can add a link to that at the end of each of your submitted articles, articles that you’ll end by including a brief *one sentence* bio (no 3 paragraph novels!!) that includes a link back to your own material and site.
If you’ve ever talked with Search Engine Marketing experts, what I’m describing here is what they refer to as a “link building strategy”, sending material to other sites and asking them to publish the material that includes the all-important link back to your own site. I’m just not charging you $2500 for the info!
The key idea here is to not just write for your own site without putting time and attention into building visibility and gaining traffic. After all, as a writer, there’s little more disappointing than writing something brilliant and never knowing if anyone ever read it.
Good luck to you!