If it's a "new manga", then you need to write an e-mail to mangago. Here is the e-mail: [email protected] (You can also find this e-mail on the below side of this site, where it says "Contact Us"). You have to provide them the download links of the chapters, so they can upload them to mangago.
Also, if it's a manga that is already here, then go to that manga, on the left side you will see a flag, click there and provide the download links. Then, it will be uploaded after a day or two.
how do i upload manga here?