1: How does one become a comic book writer?
Basically, you write comics. If you want to be a professional at it, you find editors or publishers willing to buy them. It's generally a freelance gig, not a staff position.
Depending on what part of the question you're most interested in, you might do well to track down a copy of A WRITER'S GUIDE TO THE BUSINESS OF COMICS by Lurene Haines, which goes into the business side in a fair amount of detail.
Are there certain requirements?
You've got to be able to write salable stuff. This requirement means different things to different publishers, mind you, but it's the only one that matters.
2: In your opinion, which writer would be the best person to write Superman right now? Who do you think understands the character best and could create great tales?
I don't think there's any one "best" writer for Superman -- he's one of those characters that can work well under very varied approaches, so it depends what direction you go in.
As noted, my dream team of Super-writers would include Alan Brennert, Paul Dini and Paul Levitz, but I doubt any of them are available. My all-time favorite Super-writer is Cary Bates.
But there's a lot of people who I think do a good job, from Mark Waid to Geoff Johns to Roger Stern, Len Wein, Steve Englehart, and more. I liked SUPERMAN FOR ALL SEASONS a lot, and have liked Chuck Dixon's few tales with the guy. I wouldn't mind taking a shot at the guy someday myself, under the right circumstances.
I dunno. I expect my answers would vary depending on the time of the day and the angle of the light...
kdb