Following Christ's ascension, the Holy Spirit came upon the 12 disciples during the festival of Pentecost. It was during this time that the first ever Christian sermon was given.
"But Peter, standing with the eleven, lifted up his voice and addressed them, "Men of Judea and all who dwell in Jerusalem, let this be known to you, and give ear to my words." Acts 2:14
There are a few things we can learn from Peter's first sermon in Acts 2.
First, it is Christ-centered. The culmination of his words are to proclaim Jesus as Lord and Christ. His death and resurrection are the climax.
"this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men. God raised him up, loosing the pangs of death, because it was not possible for him to be held by it." Acts 2: 23-24
"Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified." Acts 2: 26
And Peter finishes his sermon with how we are to respond to this good news.
"Repent and be baptized every one of you in the name of Jesus Christ for the forgiveness of your sins, and you will receive the gift of the Holy Spirit. For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself." Acts 2: 38-39