What do Muslims have to say to be a Muslim?

if you want become a Muslim it's simple

just say

"lailah illallah muhammadur rasulullah"

or in other Words just say the shahadah. And your heart believes it. Certain people (that are not born as Muslims) are chosen to embrace the religion of Islam.

"ash hadu ana lailaha illallah. Wa ash-hadu ana Mohammad rasulullah*