Superman came first, anyone else came later.
It all started with two North American kids - Jerry Siegel and Joe Shuster - trying out something new. Before 1938 there weren't any superheroes, only their predecessors: trashy science fiction mags and adventure-story comics. Superman's success created a new genre, and by 1941 everyone from Aquaman to Wonder Woman was out of the bag and on the newsstands.
The beginnings of superhero comics were the also the beginnings of its golden age, which lasted until the mid 50's. Superman's sucess in those early days was not only due to his novelty, but also due to the desperate nature of those years. Two major historical events are echoed, and in some cases emboldened, in the pages of golden age Superman: the Great Depression (which threatened to reignite in 1937), and more significantly, World War II.
Reading old comic books is like reading history out of the corner of your eye.
Golden age Superman mythology
Superman is the fixed point around which the world changes, and seventy years on his core character remains unchanged. In 1938 Action Comics introduced its strange invention as a survivor from a distant planet whose inhabitants are more advanced than Earth's, display superhuman strength and can jump an eighth of a mile.
The actual origin story of Superman was retold a number of times in those early years, across various comic book titles and media: Superman was born on the planet Krypton, whose advanced inhabitants neglected to heed to the warnings of Superman's parents who decide to build an emergency rocket from which to send their son off to Earth. Consistency and continuity of mythology were less important in those heady days, and as a consequence different narratives ran in parallel, especially between the comic stories and the radio show launched in 1940: Superman was either found by a couple who put him into an orphanage(!), only later adopting him, or else he arrived at Earth already fully grown, looking for a career that would allow him to learn about Earth's culture...
Yes, Superman's journalistic career, as well as his alter-ego Clark Kent, and secret love Lois Lane are present from the very beginning.
The most notable discrepancy between Superman's first appearance and now is his ability to fly, which develops as a fascinatingly gradual change. At first he can jump high, later he can fly, and in between he display's impressive acrobatics, gliding, and midair maneuvers. Some original superpowers are well known - X-ray vision and superhearing; other's less so - the ability to administer a sleep pinch, mesmerize, and contort his face to disguise himself.
The enemies of golden age Superman
Early superman is protecting America's fragile wealth from crooks and mobsters who install slot machines that cheat kids and force store owners to pay for security. Superman also faces super-villains, villains that reappear no matter how many times or how thoroughly they've been defeated; none of which of which maintained their notoriety into the present day like Luthor.
Between the late 30's and the early 40's American attitude to the war in Europe changed dramatically. Isolationist attitudes in the very early comics manifest themselves as Superman fights greedy arm manufacturers who trick (fictional) nations to join wars. The fall of France in 1940 worried many Americans, and Roosevelt stepped up military productions. Superman begins to fight fictional saboteurs and fifth columnists in pseudo-Nazi dress; by 1942 however Superman is explicitly an enemy of Hitler and Hirohito, reminders to buy war bonds and stamps appear, and many stories reaffirm the Allies' superiority and justification in the war.
Superman in simpler times
Early superhero stories are simple even by today's superhero stories' standards. Early stories are often quite prosaic, such as Superman helping failing sports players (although it does turn out that their failures are linked to blackmailing gangsters). Inevitably, as both writers and readers became acquainted with the new genre, stories become more and more incredible, as super-scientists built earthquake machines and grew giant animals.
Another trend that was introduced, to spice up the fun Superman could have, was imaginary stories, involving dreams, or my favourite, involving Clark Kent imagining what it would be like if he were in a burlesque play*.
Certain Superman motifs could only appear early comics - and I'm thinking here of Superman's early encounter with people who either have no idea who he is or think he's just a myth made up by the newspapers. But other stories weren't ready to be told, due to the simplicity of the Superman mythology. Hence there was never any talk of Superman's youth, nothing to do with Krypton, even Kryptonite itself only appeared in 1949; there was no Supergirl or Superdog; most importantly, there was no conception of stories that spanned multiple issues.
Superman at the end of the golden age
Superman wasn't created for WWII, but in truth it was his role in providing war propaganda and catharsis that justified his existence best. War did many things to Superman - it vitalized his ability to represent his nation and their needs for action. On a more practical level, it introduced chaos into a market that was still trying to settle: editors were forced to balance Superman as an epitome of wholesome goodness (telling kids to eat their breakfast and get enough sleep) with Superman as defeater of evil (telling Americans to be strong and support their troops). The war also drafted many writers and artists out of circulation, so that new faces inevitably took over on a regular basis.
These two factors - changing image and changing creators - contributed to a new sense of variation that could be manifested through Superman. The reach of this variation, however, was not stretched to its limits until after the war, when Superman's mission statement once again became unclear. 1948-58 were Superman's most malleable years, and complications not possible during the golden age proper were introduced: there was a strong focus on Krypton, what happened there, and on other (criminal) survivors. Also, Superboy was introduced; Superman's life when he was a boy. Adventures in this interim period became both more fantastic (kryptonite and kryptonians galore), and more absurd (inane plot devices) as writers tried to make coherent stories involving a man who had become all but godlike.
This set the stage for silver age Superman: the new Superman was still a superhero, but with weaknesses like any other; his greatest threats were not cunning racketeers and sly traitors, but supervillains who threatened him with all manner of tools that could take away his powers. Perhaps it had something to do with the age: the hard 30's were also simpler, people could believe and want an invincible hero; the wealth of the golden 50's destroyed the naivety which had made Superman possible in the first place, and made way for our modern era.
OK, now I want to read golden age Superman. How? Insofar as comic books are concerned, there were two titles running in parallel during this period. One titled "Superman", the other "Action Comics", which featured (but was not exclusively) Superman. D.C. have a number of anthologies that cover this period. The briefest is Superman in the Fourties and in the Fifties. Another option is D.C.'s new, and still ongoing, republishing of the old comics into book format in a series called D.C. Archives, following two Superman series: Superman and Superman in Action Comics. I read the former, but the latter would probably let you cover more time in less comics. I thoroughly recommend the Archives series; for at least the first couple of volumes of the Superman series, in addition to the comics are included samples of advertising**, filler (prose) stories, and my favourite, Superman's messages to his readers, which range from promoting exercise to poignant reminders of why American soldiers are dying overseas. You'd also do well to listen to at least a few of the Superman radio show episodes, I found them over at Old Time Radio Superman, and maybe also the first two seasons of the TV show starring George Reeves. Also, this site is incredibly informative as to the changes Superman's undergone over the ages.
* Clark and Lois walk out of the theatre, laughing at the burlesque play they've just seen, saying "Oh, how corny was that!", amazed by how simple the characters and humour was back in the "gay nineties". Although obvious, this is not something I had ever thought of: today we look back at, say, the westerns of the 60's as a recognizable entity which on one hand is an historical artifact but on the other we can see referenced in our contemporary media. Additionally we have a tendency to look back on older periods, especially in cinema, as simpler. People in the 1940's felt the same way, but their 1960's were the 1890's!
** Some of the advertising contain detailed instructions on how the child/reader might convince their parents to buy the product, while many others depend on the child writing in to receive some free item which they are to sell door-to-door in order to buy the product.