Step 1: Step 1: Come Up With a Good Idea
1) will it last? does this concept have the potential to stand the test of time? Or will it fade into oblivion with pogs and yikes pencils?
ex: Superman = Good Idea (marketable and versatile)
A Day in the Life of a Telemarketer = Bad Idea (makes people feel annoyed)
2) Is it funny? People are more likely to read your book if it makes them smile and/or laugh :)
3) Will this story keep you inspired to write more? if you want to write it, chances are there will be people out there willing to read what you have to say.
4) Does the story have a forseable ending?
side note: As a seller of comics, I will say that story lines that have already been finishes and collected into books sell better than ongoing series with no end in sight. There is something about knowing there is a good ending (or at least a satisfying one) that compels people to go out and get the next book is a series.