For many people, Manga (漫画) is the first thing that comes to their mind when thinking about Japan. The comics are omnipresent in cafes, busses, trains, waiting rooms and restaurants. People read Manga everywhere and due to their small and handy size, they can be taken wherever you go. 
The term Manga has different meanings depending on where you ask, while in Japan a Manga has to match certain style criteria to be called as such, in western countries all comics coming from Japan are called Manga. 

The ancestor of the modern Manga are short newspaper comics and caricatures, based on old Japanese art such as ukiyo-e (浮世絵). Japanese comics came a long way and are deeply rooted in the countries culture. The “comic” itself has a much higher priority in society and a much bigger readership in the population than anywhere else in the world. 
Nowadays, there are countless Mangas in countless categories, ranging from love stories to action and horror genre based stories. The most popular Mangas in Japan today are Shōnen (少年) Mangas, which are aiming for the young male readership. Shōnen stories are often about a main protagonist seeking adventure, fights and gets stronger and stronger. Example for Shōnen Manga are One Piece and Dragonball.

Mangas can be bought new in bookstores like Tsutaya, bought used in stores like Book-off and Mandarake or can be rented. 

The most popular magazine for Manga “Shōnen-jump” releases every week, featuring the newest chapters of the most popular animes and can be bought in any Konbini. 
Manga often feature Furigana ふりがな so you don’t have to know all the Kanji that might would be necessary, you should be fluent in using Kana though. 
Enjoy reading !