Qualifications
Eh, just have a computer.
Okay, so that's not true. You'll need a minimum of a bachelor's degree, or a higher level of education if you want to be competitive for the primo jobs. In that case, a communications, journalism, or English degree would be a good idea (source). Cartography—not so much.
If you have expertise in one or more particular subject matters, you may have better luck snagging a job. The broad general knowledge is nice, but if you can tack on an extensive knowledge of fly-fishing, guess who the publisher of Fly-Fishing Weekly is going to hire.
Also, you can't just have a red pen anymore. You have to know computers inside and out, and be very familiar with most common types of editing and publishing software. With so much being published online, having a familiarity with multimedia and knowing how to manipulate images, is also important. You never know when you’re going to have to Photoshop R-Pat's head onto Kristin Stewart's body.
Along the way, you should have plenty of opportunities to gather experience writing and editing for high school or college newspapers. The bonus of editing for such publications is that you get to practice working with some truly amateurish material. It's like swinging two bats while you're standing in the on deck circle.