I'm no expert and not a native speaker, but I would use "sortir" for "going out" and "partir" for "departing". So I would use "sortir" if I were talking about leaving my house to go out for the evening, and I would use "partir" if I were talking about starting a journey. With "sortir", maybe I was in a building, and I am going out of that building. With "partir", maybe I am telling you my departure time. I hope that's right! And if it is, for a mnemonic, remember that "partir" and "depart" have "part" in common.

Thank you for all the answers :) Now it is more clear.

Partir = to depart , to leave/ to walk away, to set out: Nous sommes partis pour Londres., to drive off: Je suis monté dans ma voiture et suis parti. Sortir = to go outside, to go out/ to date

"Sortir" is more "get out" or "go out", there is an outside of something/somewhere component to it. "Partir" si more "leave", "get away", "go". It's a bit more generic