Submit your work to a manga anthology magazine. Many of today's most recognizable manga like Naruto and Hellsing got started serialized in a magazine before being collected into tankobans (or trade paperbacks). Shonen Jump, Hana to Yume, and Afternoon are among the most popular manga magazines in Japan, Europe and North America.
Submit your work to a manga distributor. One of the most popular publishers today is TOKYOPOP, whose catalog includes titles like Trinity Blood, Samurai Deeper Kyo, and Rave Master. The company is also one of the leading publishers of Original English-language manga. TOKYOPOP is also famous for its annual Rising Stars of Manga contest, where the winners have their manga published in a special anthology book.
Self-publish your manga. Once you are finished creating your manga, you can either make copies of it using a good printer/copier or you can make use of a print-on-demand service like Cafepress.com. Shop your manga around at local bookstores, comic shops, and newsstands. You could also sell or give away your manga at comic conventions, using it for grassroots promotion and/or a networking tool with other publishers.