When a non-believer becomes Muslim, should he pronounce the two testimonies of faith, al-shahaadatain, or perform wudu' (ablution) first?
He should say al-shahaadatain first, then he
should purify himself for Salat and he can perform ghusl (complete
bath) because the prophet (peace and blessings
of Allah be upon him) ordered some of his companions to perform it when they
accepted Islam.
See question #179 for an explanation of the two
testimonies of Islamic faith.