When the number might not be recognized as a year, we say "The year" first. I would say these as
"The year five fifty-one BC" and
"The year twenty-one AD."
"BC" and "AD" are, as you'd expect, pronounced "bee cee" and "ay dee."
There's another detail, though. "AD" stands for "anno Domini," i.e. "year of Our Lord," i.e. Christ. "BC" stands for "before Christ." Both assume a framework of Christianity. Scholars and historians prefer to use the culturally neutral terms "CE" (Common Era) instead of AD, and "BCE" (before Common Era) instead of AD.