Over 500 believers saw Jesus after His resurrection. (
1 Cor 15:6)
About 120 of these (approximately one fourth of the infant church) were in Jerusalem "continually in the temple praising and blessing God" (
Luke 24:52-53)
They (we assume the 120 but some think it refers to the 12 apostles only) were all in one accord and in one place when the wind and fire came and they began to speak with other tongues. About 15 languages are mentioned that were understood by different people.
Peter stood up and preached a message in a commonly understood language.
When the Holy Spirit convicted people, they asked what they should do. Peter told them to repent or turn from the sin of rejecting Jesus as Messiah, to wash in the mikveh baths in the temple because their sins were forgiven, and told them that they too could receive the promise of the Father which they had seen and heard.
We are told that about 3000 Jews received the Word (Jesus) and were immersed. We don't read about how many, if any, received the Holy Ghost baptism.
Some folks (called Dispensationalists) believe that the 120 were not really saved until that day and that this was the beginning of the New Testament Church.