Determine what kind of story you're going to tell with your comic book. Most comics embrace superheroes, but you can also create comics that feature mystery stories, action adventures, romance, and even comedies if you wish. The genre plays a role in the characters you create, so begin with a basic sense of what your comics will be about.
Come up with an idea for a central protagonist: the hero at the center of your comic book. It needn't be very elaborate, just the basics of who he is and what he does.
Flesh out the details of your hero in order to give him a sense of a personality. If you're writing a superhero comic, describe his powers and weaknesses. If you're focusing on a different genre, state what the hero does and how he got into his current position. Write down a few paragraphs covering his history, his personality and his demeanor, as well as a detailed description of any special abilities he possesses. Try to keep all his traits thematically unified, instead of just tossing attributes in at random.
Draw a picture of your hero, garbed in the outfit in which he will most often appear in the comic. If you're not artistically inclined, you can use an online program like HeroMachine to create an image for you. Alternative, you can just write down a physical description, though if you intend to actually create a comic book with this character, you'll eventually need to draw a picture of him.
Repeat Steps 2 through 4 with every other principal character you intend to include in your comic. Your hero will need villains to find, each with their own background, personality and physical look. He'll also have supporting figures such as partners, sidekicks, friends and romantic interests. Each of them should have a personality write-up, a history, a list of powers and/or abilities and a picture to cement his or her image.
Organize your pictures and write-ups in a single location, where you can refer back to them when necessary.