Who did you choose?  Nick
Why is it easy to write about him?
Nick has gotten fun for me to write in the last few years, partly because I started to really like him, and partly because, like others have said, he does seem very deep and multi-faceted, and you can write him in a number of ways and put him in any situation.  He's good for comedy and for angst.
What do you write about romance, comedy, drama etc?
Mostly drama/angst, with a hint of romance.  Sometimes suspense or comedy.
Who is it hard to write about?  Howie and Kevin
Why?
Well... I hate to say this, but it might just be because they're not my "favorites" from the group.  Don't get me wrong; I love them both.  But Brian has always been my favorite, and when I first started writing fanfic, pretty much all my stories starred Brian.  Then I branched out into AJ, who was my second favorite, and then I started to like Nick and started to write about him.  I like to write about the guys I like best.  And although I like Kevin and Howie a lot, I've just never been moved to write a story just about one of them.  They're always in my stories, just usually not in a starring role.  I think that's the honest answer.
Who and what was your first fan fiction about?
My first fanfic was about Brian.  I always go back and forth about which story to call my actual first, cause my very first was never finished and never put online, but I rewrote it later and finished it then (although it was much different).  It's called "Years of Grace," and it's sort of like Brian's life story, from age 5 through the present, but with a fictional romance twist.  My first finished/published fic is "Heartache," and all I can say about it is that even though it's not very good, I still like the idea of it and would rewrite it if I had the patience.