The Disney era canon has yet to officially confirm any details of the Emperor’s backstory. All that can be said for certain is that he was a prominent politician on his homeworld of Naboo, and that he was ultimately appointed as the planet’s representative in the Republic Senate. Unbeknownst to the galaxy, he was actually powerful in the Force, and he somehow caught the attention of the Sith Lord named Darth Plagueis. Palpatine himself was given the Sith name «Darth Sidious.» Everything Palpatine did, every detail of his career, was secretly to serve the interests of the Sith.
Interestingly, though, the old Expanded Universe contains information about Palpatine that may still be relevant. James Luceno’s novel Darth Plagueis told the origin of Palpatine, revealing that he was born into a noble Naboo House and slaughtered his own family as part of his initiation into the Sith. While Disney declared most of the EU non-canon shortly after they purchased Lucasfilm, this is something of an anomaly, simply because the book was published long after the acquisition. What’s more, Luceno’s new canon novel Tarkin heavily alluded to characters and events from Darth Plagueis, so something similar may well have happened in the new canon too.
Alternatively, it’s worth noting that there’s another popular theory: that Palpatine is far older than anyone has ever realized. This could be supported by Star Wars: The Rise of Skywalker, which hinted that a dying Sith transfers their essence over into the body of the Apprentice who has slain them. If this has always been the case, then for millennia there has only ever been one Sith Lord, growing in power, possessing host after host, gradually tilting the Force out of balance. Palpatine was simply the latest host.