Understand the unique qualities of the comic book. They come out every month (approximately), so you must pace your story in a satisfying way. The end of each issue should have its own end that also encourages the reader to pick the book up the next month. Further, you have a little more freedom than novel and short story writers. The images you create will actually be seen by the reader, not just coaxed into life by their imagination.
Consider a couple striking images you will have in your comic book during your brainstorming session. While you can't fill every panel with something amazing, you should give the artist at least two or three opportunities to produce something memorable. These images should also advance the story you're trying to tell and deepen the character you're using.
Provide the artist with the description he or she needs to draw what you have in your head. If you just want your character to be in a kitchen, and you don't care what the kitchen looks like, that's all you have to write. Be more detailed if that's what you want: "The kitchen is messy and looks as if Bob hasn't cleaned it in years. Dirty dishes are stacked on the counter beside the rusting refrigerator...."
Employ crisp, efficient language in your script. In a comic, unlike a novel, the reader is not able to really get into a character's mind. Where a novelist can write something like, "Bob was really sad because of his problems and his heart was sinking into his stomach," a comic writer must communicate this visually. You could tell the artist "Bob looks hurt. His lips are puckered and his eyebrows are creased as a tear tracks down his cheek."
Follow the correct format for writing comic books, at least if you're going to show your scripts to anyone in the industry. You can write them however you like for yourself, but a professional or serious comic book enthusiast will take to your work better if you follow the format. Much like a film screenplay, the action should be laid out on the page in wider paragraphs, with the dialogue centered under capitalized character names. See the formatting link under the "Resources" section for an example.