he has a bunch of shorter stories that you could easily jump into without any order but i'd recommend reading uzumaki first. its a classic, is one of his longer mangas, and encapsulates a lot of what makes ito's stories unsettling. i've never actually gotten one of the short story collections so i can't recommend any in particular, but i've yet to read an ito manga i regretted reading so i would say just pick whichever one interests you the most.

also, if you're into cosmic horror, i recommend hellstar remina since i don't see it talked about as often as his other stories.

If you've ever seen that "This is my hole" meme, the story Enigma of Amigata Fault is where that originated from. A personal favorite of mine is Long Dream. And a longer series of overarching stories called Uzumaki is great as well.