What dua protects me from shaytan?

The basic words of refuge

"A'udhu billahi min ash-shaytan ir-rajim" is the first protection every Muslim learns. You say it before reading the Quran, when a bad thought hits you, and when you feel anger rising. Two men once argued in front of the Prophet ﷺ, and he said that if the angry one said these words, his anger would leave him (Sahih al-Bukhari 6115).

The Quran also teaches a longer version, which is lovely to say when you feel under pressure:

Ayat al-Kursi and the last two surahs

• Ayat al-Kursi (Quran 2:255). In a well-known narration, a thief told Abu Hurayrah that whoever recites it before sleeping keeps a guardian from Allah with him and Shaytan cannot come near him until morning. The Prophet ﷺ confirmed it was true (Sahih al-Bukhari 2311).
• Surah al-Falaq and Surah an-Nas (Quran 113 and 114). Both are literally prayers for refuge, the second one specifically from "the whisperer who withdraws". Many Muslims read them, with Surah al-Ikhlas, in the morning, the evening and before sleep. See What should I say before sleeping?

Everyday moments of protection

• Coming home: say "Bismillah" when you enter and when you eat. The Prophet ﷺ said that when a person remembers Allah on entering and at the meal, Shaytan tells his companions there is no place to stay and no dinner here (Sahih Muslim 2018).
• Your home in general: recite Surah al-Baqarah. The Prophet ﷺ said Shaytan flees from a house in which it is recited (Sahih Muslim 780).
• Morning and evening: the daily adhkar are a shield in themselves. See What are the morning and evening adhkar?

No need to live in fear

Islam teaches that Shaytan's main weapon is whispering, not power. He cannot force you to do anything, and remembering Allah weakens him. Converts sometimes worry about jinn or possession after reading online stories, but a believer who prays and keeps up these short words has strong protection. See Can jinn possess me after I become Muslim? If you feel constant fear, intrusive thoughts or low mood, it is also wise to talk to a doctor; that doesn't conflict with dua.