As we examine Scripture, one of the things that become clearer and clearer is God's sovereignty. But this is not presented in some abstract sense at all. "God is in control" has, unfortunately, become somewhat of a mantra... the thing one says when they feel helpless. The truth is, however, that Jesus really is in control. A simple reading of the Bible makes the case that God is very much interested in the affairs of His creation.
George N. H. Peters (a Lutheran minister in the 1800's) coined the term THE THEOCRATIC KINGDOM. That is probably the most succinct, albeit somewhat ambiguous, phraseology to date. Basically God created all things and, by virtue of His continued interaction with creation, rules over it. God then delegated certain areas of management (stewardship if you will) to man.
(Note: this is the crux of the dispensational framework we operate under.)
Because of man's failure to comply with the over-arching THEOCRATIC KINGDOM (rule), The Lord has changed the status of the servant as well as the scope of his/her/their domain. Life, for the managers, got more and more complicated and dangerous, but to be fair... it was a self-inflicted wound.
Flash forward a millennium or two: God called Abram out of Ur to form (what would become) a counter culture. This nation would demonstrate the THEOCRATIC KINGDOM to the other nations. As you may know by now... they failed. Israel went into captivity. God, knowing full well that they would fail, provided hope through the promise of a Savior (The Messiah) many years prior to the aforementioned failure. As the managers continued to fail (again, the crux of the dispensational framework), the identity of the Messiah came into greater and greater focus.
Now... here is the tricky part... when the Messiah (Jesus) was revealed, He started talking about the Kingdoms. Kingdoms... you mean, plural? Yes.
Jesus taught about the Kingdom of God and the Kingdom of Heaven. What exactly did Jesus mean in John 18 = "My kingdom is not of this world"? Certainly this Jesus is the same Son of David that will rule and reign on the throne in Israel... right? Yep... one in the same. Jesus taught two different groups about two different Kingdoms and that is where most of the confusion (as far as I can tell) comes from.
To Israel, Jesus made an offer of the Messianic Reign of the Son of David. That Kingdom is one of righteousness and justice (hence the content of the "Sermon on the Mount"). After His rejection by the religious elite of His day, Jesus, knowing full well about the coming of the crucifixion, turns His teaching to the more immediate... the church. The organism that is to function as the ultimate in creation-management... the one with Messiah operating as the head (thus explaining all this business about Peter being a fisher of men and why the "kingdom is like a mustard seed").
There it is, in a nut-shell... If it seems rant-ish, I apologize.
So there you have it... we have discussed the "what?" of the Kingdom.
The question now is... "what am I supposed to do now"? And that has an easy answer: Follow Jesus