If you have a claim on the kingdom, you can press your claim.
If the target king is a heretic of your religion, your religious head might declare a crusade against the king.
If you are invading realm that is larger (more holdings) than yours, you can use the Invasion CB.
As mentioned by svick, if you are an emperor, you can press a claim on the kingdom title for a relative or vassal, and upon winning, the claimant will be your vassal.
Ways to literally invade the entire kingdom at once (You can break a truce, but the penalty is harsh) There are a couple of things that make conquering a kingdom hard.