Formal: Sut mae'n mynd? (pronounced "soot mayn vung")
Informal: Sut mae? (pronounced "soot may")
You can also use Sut wyt ti? (pronounced "soot vut ti") for an informal "how are you" but it is considered slightly less common.
It's important to remember that the Welsh language uses gendered pronouns, so "Sut mae" is used for both men and women.