This passage is broken up into two sections:
Last We Talked...O Theophilus (vss 1-5)
Last We Talked...To Jesus (vss 6-11)
In the first section, St. Luke (who is the common author of both the Gospel of Luke and the book of Acts) is recalling what the purpose of the "first book" (the Gospel of Luke) was about. It was about what Jesus BEGAN to do and teach. In the book of Acts, we have an account of what Jesus CONTINUES to do through His church. In this passage, St. Luke sets up what is to follow in the rest of the book.
In the second section we have a narrative of the final moments of Jesus' life on earth. Why bother putting this story here if the book of Acts is about what the Apostles do? Here we have an enthronement story. Jesus ascends (in a narrative calling to mind the transfiguration, Elijah's ascension, and especially the vision of Daniel in Daniel 7) and sits at the right hand of the majesty on high (Heb 1:3).
Christ's enthronement means that He is the head of the church (Eph 1:22) and is filling all things (Eph 4:10) until all His enemies are put beneath His feet (Ps 110:1). This means, further, that every area of our lives are under His rule and reign! He both upholds us and sustains us (thank God!) and commands us to put all areas of our lives under His rule.
Structure of the passage:
A. Jesus presented as alive (vss 1-3)
B. Do not leave Jerusalem (vs 4a)
C. You will be baptized with the Holy Spirit (vss 4b-5)
D. Will you at this time restore the kingdom to Israel? (vs 6)
C.’ You will be baptized with the Holy Spirit (vss 7-8a)
B.’ Leave Jerusalem for Judea, etc. (vs 8b)
A.’ Jesus presented as ascended (vss 9-11)