Teenage Mutant Ninja Turtles (or TMNT for short) was a self-published comic book created by Kevin Eastman and Peter Laird in the early 1980s. Combining elements of Marvel’s X-Men and Daredevil, Frank Millar’s Ronin, and Dave Sim’s Cerebus, TMNT is the story of four mutant turtles taught how to be ninjas by a mutant rat who then decide to defend New York against numerous threats while they remain sequestered in the sewers underneath the city. It was by far the most popular self-published comic book at the time, spawning numerous copycats and founding a media franchise that included toys, cartoon shows, movies, and numerous other merchandise, often with only peripheral connection to the original source materials.
The first 17 issues of TMNT (barring the Cerebus crossover) have been placed on the official website along with a few early one-shot spin-offs. To see them, go to the “Comic Book” section at The Official TMNT Website and scroll down to the bottom of the page.