It depends. If you read manga that is not licensed in the United States (assuming you are North American, sorry if you're not), then it's not outright illegal...well, not unless you get extradited to Japan where you will face prosecution. Likelyhood of that? Less than zero. Same with the scanlation groups.
You aren't denting sales that go on in Japan/Korea/China and they aren't recieving any revenue from US sales. The only damage they receive is potential sales when/if (the IF is huge, think of it in ten foot letters), they decide to license the manga in your region.
IF on the other hand the manga you are reading is licensed, go buy the tankoubon from the bookstore. Not only does it give the publisher money, but it also tells them to keep translating and pushing out future volumes of the series; I can't tell you how many manga have been dropped while US publishers sit on the NA license, so no-one can safely scanlate.
There is currently only a few ways to pay for and read manga online. Western comic book companies have adopted this model, and they have streaming Anime services like crunchyroll, but only the big stuff like Shounen Jump and Yen Press for manga as far as I know.
Sorry, but for the most part if you want to do this legally for licensed Manga/Manhwa, you have to go to the bookstore. On the flipside, most Manga is not licensed for any other region besides Japan, so you can read those scanlations however you like, at least partially mollified in the knowledge that you CAN'T pay the artist, even if you would like to.
