To become a martyr you have to die for something you believe in such as a religion.
Tradition says that St. Peter was crucified upside down in Rome about the year 64-65 AD.
He was stoned to death for preaching about Our Lord.
Most martyrs are saints.
St. Andrew, the first apostle, was crucified on an X shaped cross in Greece. We do not know the date but it was probably in the middle to late first century.