It is usual for an author to begin his book with a preface that enables the reader to see where the author has come from and what he intends to say. Chapter 1:1-8 of Acts functions like a preface and in it Luke details what his main emphases are going to be in his book.
His first point is that the risen Jesus is still active, although he is no longer present physically on earth. I think we can deduce this from the verb ‘began’ in verse 1 – it suggests that he is still continuing with his work although in a different way. Luke is saying that the contents of his second volume will describe the ongoing acts of Jesus as he oversees the extension of the kingdom of God.
The second point stressed by Luke as he writes to Theophilus is that the Holy Spirit will be active once he comes. In contrast to the rulers who were responsible for the development of earthly kingdoms, and who used fiendish power to defeat and enslave their enemies, the risen Jesus will use heavenly power (the Holy Spirit) to bring spiritual life to sinners who are the enemies of God. The Book of Acts will detail what took place in several places when the Spirit came in his power.
A third point that Luke stresses is that the blessings connected to the growth of the kingdom of God will come to a spiritually-active church. In Chapter 1, he describes what the church did as they waited for the Spirit to come; in subsequent chapters, he will detail what the church did after the Spirit came. What is important to note about this point is that God blesses those who practice their faith.