Find a type of manga that you like the most. There are so many different genera of manga it's ridiculous sometimes. There are standard action titles like "Yu Yu Hakusho." A great fantasy title is "Inuyasha." Manga isn't just for one gender, there is something for everyone. If you enjoy romance, a good title is "Captive Hearts."
Look up the publishers of manga. To read the manga online legally, you'll have to read the manga straight from the publisher's website (see the "Warning" section of this article for more information on legality).
Navigate to a manga publishing website after you've found one you like, and read their online manga. There are plenty of manga publishers in the world with some of the largest being Shonen Jump/Shoujo Beat, Del Ray and Tokyo Pop. There are a couple examples of manga publishers that post manga online legally in the Resources section of this article.