Percy Jackson and the Lightning Thief is book one. It's the logical place to start. You probably don't need to read all the other books in order, except for The Last Olympian read that one last. Oh look! There's a new one!

It's been about a decade since I read these books. I might need a refresher.

Start with the lightning thief as someone else suggested but fair warning it is still a YA novel. The author is great but you're still gonna find it a little more predictable than you may have as a kid