I'm far from an expert, but I think comics are great fun to make and really want to get better.
First of all you've got to have an idea of course, a story to tell. For my part, there are loads of stories within the story that we're working on, for example from the childhoods of our characters. Just make sure it's a story you like, since you've got to spend quite a lot of time working details!
As in any story, character is important in comics. Just that in a comic, the appearance of the character is crucial. I've found that it really pays off drawing all the characters separately a couple of times, just to settle on a look.
Here's an example from my current project:
The character is the older brother of my main character Adreana, he's called Fredrico and the son of the local mafia boss, the Don. This has made him pretty over-confident and, well, a good old-fashioned bully.
Sonny Corleone (From 'The Godfather')
Turiddu (character from the opera 'Cavalleria Rusticana')
That's all for now, next time it's all about setting :)