Jules Verne is probably the first author of what we would consider science fiction. He wrote in the late 19th century, sometime after the US civil war. His best was "20,000 leagues Under the Sea". Verne wrote in French, and I still remember the kinda shabby English translation I took to summer camp one year. Verne's prose was probably only mediocre in French, and was down right miserable in English translation. But the story was gripping enough to overcome weaknesses in the writing. Disney made a good live action movie in the 50's, with James Mason as Captain Nemo and Kirk Douglas as Ned Land. Technicolor, with a fine Nautilus and great underwater shots. Not for nothing did the US Navy name their first nuclear sub Nautilus.
Next in line was Edgar Rice Burroughs, the creator of Tarzan. ERB's first published story was "A Princess of Mars" back in 1912. John Carter's derring do, deadly sword play and beautiful Martian princess set the style for space opera that lasted up thru Star Wars. Princess Leia inherited a lot from Dejah Thoris. The sand people riding their Bantha's look pretty much like Green Martians riding their Thoats. Burroughs followed up with about ten more Martian stories over the next 30 years. The first three are the best, the later ones are pot boilers.
Edward Elmer Smith (EE Doc Smith) first story "Skylark of Space" was published in the 1920's. It was "super science". Lots of high tech (for the 1920's) stuff, powerful space ships, resourceful scientist/engineer heroes, pretty girls, evil drug runner bad guys. EE Smith kept publishing right up to his death in 1965. I'd rate his stuff good for kids but a little corny for today's grownups. I encountered Doc Smith as a kid and still like him.
There were plenty of other science fiction writers back in the day, but these three are my favorites from the era before John W. Campbell.