I think most guides overcomplicate this. They tell you to start with 'The Lightning Thief' from Percy Jackson and the Olympians, and they're not wrong, but they skip the why. That first series is the foundation for everything—the characters, the modern myth logic, the humor. If you jump into the later stuff like 'The Trials of Apollo' first, you'll be bombarded with inside jokes and returning heroes that won't land.
After the original five Percy books, I'd slide right into the sequel series, 'The Heroes of Olympus'. It continues the story with most of the same gang, plus new faces. Then, and only then, would I branch out to the spin-offs like the 'Kane Chronicles' or 'Magnus Chase'. They're set in the same world but with different mythologies, so they work as standalone breaks. Reading them in publication order keeps the surprises intact, especially the crossover short stories Riordan slips in later.