are you looking to sell actual printed manga online or just host manga for users to read?
It is a big difference how on you monetize that. If you are looking to sell printed materials, you need to make it prevalent enough to compete against amazon. Maybe stuff that came straight from Japan, or right of the newsstand type stuff... but that would be a hard market to break into.
If you are trying to host material for free, well you have one big concern. Copyright infringement. Look into what your nation's copyright laws are and what you are allowed to do with other people's work.
After that, how you monetize is by selling ad space. Google Adsense is the most popular. Build your site, make the material easy to access and where users WANT to visit, and then create a space for ads on your site design. It'll be very rough at first, with very little money, and it may never actually become more than a few dollars here and there, but that is the most common way to make money with sites online.
Good luck