In this chapter, Manasseh leads Jerusalem into idolatry.
The very first thing I should say is that this is not, of course, the same Manasseh that we read about in Genesis. There is a "tribe of Manasseh", who are one of the twelve tribes, descendants of Manasseh the son of Joseph (see e.g. Gen 41:51). This is another Manasseh, a worse Manasseh, if you will. Not to say that the first Manasseh was much of a saint (if the other sons of Jacob are anything to go by), but this one is clearly worse. This is a Manasseh who is steeped in idolatry and murderous tendencies, and both the author of Kings and the unnamed prophets in v. 10 rebuke him in strong terms.
The second thing I want to point out is how early Manasseh became king and how long he reigned. He was king from age 12 through 67, which is 55 years. By comparison, Hezekiah was king for 29 years and even David (the most famous king of Israel) reigned for only 40 years. It's hard to overstate the significance of this, because when a single person is king for more than about 30 or 40 years, it means that an entire generation is raised up that does not know life before their current king.
Regents like this have the capability of shaping an entire generation from their youth to adulthood, and it means that their personality and character can be stamped onto a culture in a way that does not happen when a king reigns for less time. I already mentioned how much power ancient kings would have to shape the society under them (and that is certainly true), but the difference is that when they reign for long enough to shape one generation, they can institute cultural patterns that generation will pass down to their children and descendants beyond that. It means that a king can extend influence beyond his own lifetime.
In the case that a king is righteous (like David), this is good. But in the case that a king does evil, it magnifies the depth and durability of that evil. Unfortunately, Manasseh is the second kind, and most of this chapter is dedicated to telling us about all the evil things that Manasseh did. Principally, that is Manasseh's idolatry and shedding innocent blood.
Verse 13 uses a peculiar expression, where the LORD (through his prophets) says that he would judge Jerusalem with the "line of Samaria and the plummet of the house of Ahab". This is basically using an analogy from how people would construct houses. When building a house in biblical times (and still to this day, actually) people would use a plumb line, which is just a line of cord with a weight attached at the bottom. The reason why is that you can use this line to determine a straight up and down direction, so when building a wall it lets you build a wall that is exactly parallel to gravity, which makes the wall stronger and less likely to fall over.
In this chapter (and elsewhere in the OT), the plumb line is used as a metaphor for God's judgment because God is essentially saying, "your wall is crooked and I am going to use this plumb line to show how you are defective." It shows the people what they are supposed to be like, but they are not. The plumb line means that Judah has "built itself incorrectly", in a moral sense, and God is going to judge them because they have not done what is right. In this particular verse, I think the prophets call it the "line of Samaria" because Judah is sinning the same way that Samaria sinned, and as a result Judah is going to suffer the same judgment that God inflicted on Samaria.
Amon reigns after Manasseh, but Amon is a much less consequential figure because he is assassinated shortly into his reign. We are not given the reason why Amon is assassinated, but just the fact that it happened seems to indicate that there is growing instability in Judah. As we see more violence in Judah and the kings are more and more consistently seeking evil and serving Baal, it is clear that we are approaching the very end of Judah's independent kingdom. The Babylonians invasion that was foretold in the previous chapter is swiftly approaching.