If you have access to a concordance, you can look up all references to “kingdom” in the New Testament. Most of them will show “kingdom of God” as the full phrase. A lesser number (and mostly in the gospel of Matthew which was written more for Jewish readers who were more used to the circumlocution) will show “kingdom of heaven.” And there are a scattering of other variations (including simply “kingdom”). Be aware also that some of these references are to the word “kingdom” without reference to God’s kingdom. For example, Jesus referred once to Satan’s kingdom.
Since the kingdom was Jesus’ most frequent theme, you will find many, many passages for reflection or study.