How do I say 'you're welcome'?

不客气 is the safe, correct response to 谢谢 — it works everywhere and offends nowhere. But it is worth knowing that native speakers deploy a richer set of responses depending on the relationship and the size of the favor:

• 没事 (“it’s nothing”) minimizes your contribution. Use this for small favors — holding a door, passing something, giving directions. It says “this cost me nothing, don’t worry about it.”
• 应该的 (“it was only right”) is warmer than 不客气. It suggests that helping was natural given who you are to each other. A colleague might say this after staying late to help; a friend after lending money. It turns the thanks into a statement about the relationship rather than the transaction.
• 不客气 is the neutral middle — polite, complete, and slightly formal. It is the right choice with strangers, in service situations, and whenever you are unsure.

Chinese also has a fourth option that English lacks: saying nothing and simply moving on. Between very close friends and family, a verbal 谢谢 / 不客气 exchange can mark unnecessary distance.