Pioneered by Stan Lee, "Marvel style" scripts present the entire story in condensed form, allowing the artist to set the pacing. Dialogue is added after the art is done.
Full script style breaks down the story into individual pages and panels, giving detailed directions to the artist. Dialogue is included in each panel.
The beginning of the story introduces the hero and the main problem; the middle shows complications to the story problem; the ending resolves the problem.
Most (but not all) comic books are 22 pages long. Each page is divided into panels, or boxes, with art and usually dialogue in each.
Spoken dialogue is told in balloons. Thought balloons have generally dropped out of usage, replaced by captions (boxed text).
The page with the title and credits is called the splash page, usually having a single panel.
Every comic book publisher has different formatting needs, so be sure to check with each individual publisher before submitting.