The eight page story "Swamp Thing," scripted by Len Wein and with artwork by Bernie Wrightson was first published in the DC comic House of Secrets #92, cover dated July 1971. The story spawned a stand-alone Swamp Thing comic book of which Wrightson drew the first ten issues. The series has long been regarded as one of the best comic books of the era and the peak of Wrightson's tenure at DC.
TwoMorrows published a special issue of Comic Book Creator called Swampmen with an extensive interview with Wrightson about the creaton of Swamp Thing.
Below are scans of the original art for the first Swamp Thing story from House of Secrets. The scans are from the Heritage Auctions web site. Below that is a list I compiled of all the places the story has been reprinted because I like that kind of thing. If I missed anything let me know in the comments and I'll add it. I'm sure there's some foreign editions I missed.
The Saga of the Swamp Thing #33 (1985)
Roots of the Swamp Thing #5 (1986)
The Saga of the Swamp Thing Vol. 2: Love and Death (TPB, 1990)
DC Silver Age Classics: House of Secrets #92 (1992)
Swamp Thing: Dark Genesis (TPB, 1992)
Showcase Presents: The House of Secrets Vol. 1 (TPB, 2008)
DC Comics Classics Library Vol. 4: Roots of the Swamp Thing (HC, 2009).
Wrightson's Swamp Thing 1: Alec Hollands Saere Skaebne (Dutch, 1979).