Acts 2 - Nathanael

 "Therefore let all Israel be assured of this: God has made this Jesus, whom you crucified, both Lord and Messiah."

37 When the people heard this, they were cut to the heart and said to Peter and the other apostles, "Brothers, what shall we do?"

38 Peter replied, "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. 39 The promise is for you and your children and for all who are far off—for all whom the Lord our God will call."
Acts 2:36-39

Peter's first sermon was quite powerful and full of the Spirit of God.  He spoke boldly and didn't hold back the truth ("whom you crucified") and the Holy Spirit worked through him to touch the hearts of many people ("what shall we do?").
His call to repentance and baptism still ring true today.  We need to recognize our wrong ways and turn to what is right and follow Jesus.  

They devoted themselves to the apostles' teaching and to fellowship, to the breaking of bread and to prayer. 43 Everyone was filled with awe at the many wonders and signs performed by the apostles.
Acts 2:42-43

God continued to work through the apostles.  Those who began to follow Jesus devoted themselves to following by devoting themselves to 1) the teaching of the apostles - this required them to be with the disciples regularly 2) to fellowship - this required them to be together with other believers regularly 3) breaking of bread - they ate together regularly and 4) prayer - they prayed regularly.

This sounds like a good list of items for us to follow as well!