MangaFox is probably the most common site to read manga, and for good reason. MangaReader, Batato, MangaEden and Mangahere are all decent too. I personally prefer http://www.taadd.com/
If you want an app, you can use the MangaReader app as suggested in the answer, but I'd personally suggest using an app called Manga Rock - it allows you to access several different manga sites on your phone, so if you can't find something on one, you can check a different source. It has all the main Manga sources. It's free to download, but you need to pay some money if you wanted unlimited downloads.