I find planning very important. I've re-written my story about ... 6 times now
, but I was much less experienced when I started.
If you're rather new to comic writing, I recommend just getting some paper, some pencils, coming up with a fun short story to tell (it can be retarted, it really doesn't matter) and just going at it. Panel layout and just learning to get the "camera angles" you want are pretty important and you learn FAST.
also, for the "real" story you want to write, as Jack said, it's *very* important to know your characters. Think long and hard about who they are. Even if you don't write out their past in your book, know their past to the very last detail. If you have solid characters that you know like yourself, and you can imagine how they would react to a certain situation or say to someone else, the story will write itself
Just make sure to have a "skeleton outline" of what you want to happen in each chapter and what themes you want to present and so on.