That's a lot of complicated questions, but that's what we're here for. I'm going to focus on the high points and try to hit all the questions rather than drill down in all places equally. That can always be for follow-ups.

How was the structure of the Confederacy determined, how did they decide who would determine it and did the USA have any idea it was happening until the Civil War started?

They copied the US Constitution, with a few modifications, and made it an explicitly and more radically proslavery document. That happened at a convention attended by delegates from the six Lower South states that had already rebelled, though they expected Texas to come along directly and so wrote in a special provision for it. All of this happened very much in the public eye at Montgomery, Alabama and as the culmination of an extremely public political movement. The idea of individual states going out and hoping to make it on their own had been floated before, but it was generally considered unsustainable unless at least a few states went out together.

It happens that the states which rebelled did so by individual actions despite all of that, but they did so with a reasonably good idea that they would not be stuck twisting in the breeze. An informal assurance to that effect might even have been decisive in South Carolina getting the ball rolling. Most of the state's political class wanted to try, but they were very cognizant that during the Nullification Crisis they got close to the brink and no one came to pull their fat from the fire. A new railroad between Charleston and Savannah had just opened at the time the state was in convention on rebellion and they received explicit assurances from Georgians who had come up to town on the new line to celebrate that their neighbor would be along quite shortly. Was that really the tipping point? History didn't run the other way so we can't know. William Freehling thinks it was, but I don't know that many have rushed to follow his lead in the decade plus since he published the argument. Contingency is obviously an important thing in history, I think he leans on it a little too hard here.

I'm mainly trying to understand where/when did the idea of secession begin. And when Fort Sumter was getting shot, what were members of Congress doing? Were there southern politicians who were caught off-guard? Any northern ones who sympathized with the CSA?

Secession from the US is something you can read about in the sources going way back, arguably to ratification of the Constitution. Doing that requires a fairly shallow reading, though. It's important to consider disunionist talk in its particular contexts, few of which amount to an actual policy preference or program of action. Instead it's usually a threat: "if you do this, we are taking our land and going home!" or a prophecy meant to be averted: "the way things are going, the Union cannot endure". Most discourse on the subject is rhetorical or theoretical up into the 1840s, by which point the white South has begun to see the North as increasingly hostile to the continued expansion and thus prosperity of enslaving people. They're way ahead of actual Northern opinion here, but have recognized a real trend of antislavery politics gaining ground. That gets them into a kind of vicious spiral where they seek more and greater concessions, which is also spurred by their slow loss of majority control of the federal government that's going on contemporaneously. That proves to more and more wavering white Northerners that the South actually is a hegemonic power bent on turning free states into de facto slave states -a point which they are dead on about, at least functionally- and thus kicks them closer to the antislavery column. Rinse and repeat until the cannons go off.

There had certainly been sectional crises before the 1840s. Arguably US political history is a big one in general. What's different about the late Antebellum is that the crises last longer -years on end- and their acrimonious ends don't stay ended very well. After the Compromise of 1850 gave the white South nearly everything asked for, politicians consumed prodigious amounts of alcohol -ok, they did that all time but they did it more in September of 1850- in the name of finality. They would never have to talk about slavery again. Cue a couple years of high profile rescues of fugitive enslaved people, mass protests, Uncle Tom's Cabin, and then a whole new sectional dustup. Past crises tended to last a few years at most and have a decade or more of padding around them. This time it just never really ended. Even attempts to declare it all ended were understood by some, rightly, as keeping on keeping on with the arguments. Effectively, the US enters a steadily escalating sectional crisis around the annexation of Texas and doesn't come outof it until 1876 or so.

That said, to loop back to your question, no one is really surprised that something happens. There were certainly many politicians in the North who thought the South was crying wolf and even that once the immediate explosion of angst at losing the presidential election settled down they'd be back with hats in hand. The Republicans went into Secession Winter with a whole heap of unfounded optimism about the white South cooling off. Southern politicians knew better. Obviously we can't speak to every single person and probably some were in denial right up until their state rebelled around them, but in general they did not miss where things were likely headed. Nor could many of them, since there were threats of secession back in 1851 serious enough to require major speaking tours in response and rather fragile proclamations to the North on the conditional nature of Southern loyalty by the guys who went on tour for the Union.

So far as Northerners who sympathized with the CSA, there were some. They're much more common in free states that border the South. They include a sitting Senator who was expelled from the Senate for treason, basically. Said Senator had owned a slave labor camp in Kentucky for ages by that point, but that fact in itself wans't necessarily dispositive; Stephen Douglas of Illinois owned a 20% share in a similar camp in Mississippi courtesy of his wife's dowry and was all on board for fighting hard to save the Union. During the war, Northerners willing to aid the South are called Copperheads. It's a double insult because they're poisonous snakes and they don't have the good decency to give you a warning rattle.

If you have any books on the planning of the Civil War, I would love any recommendations as well.

There's planning and then there's planning. Probably nobody had drawn up a clear blueprint or a WWI-style plan for what to do, how to do it, and when for use in the event of secession. The actual events were improvised as they happened, to the point where they sometimes outpace the politicians managing niceties like actually seceding before seizing federal property. What planning does take place is chiefly rhetorical and theoretical. Politicians arguing for something in prominent venues makes it more thinkable than it might otherwise have been and that's not nothing. Likewise when they draft rambling, not very consistent and deeply incoherent theories about the nature of the American state to make everything they intend legal and constitutional.

That in mind, most of what's out there is political history. If you want a grand survey, Freehling's Road to Disunion (2 vols) is the place to get it. I disagree with him on some points and in some ways I think he's very old fashioned, but for a detailed look at Southern politics from Revolution to Secession, he's the historian. Much shorter verions include Cooper's Liberty & Slavery, which is a good starter book and a ripping good read if you're into political history, and Varon's Disunion!, which is good for parsing out the various ways disunion is used in period discourse.

A particularly illustrative case study is Freehling (again) in Prelude to Civil War. It's just about South Carolina and just the Nullification Crisis, but he has an invaluable chapter on nullificationist and secessionist thought and its general incoherencies. It's also a classic monograph, even if it shows its age in how much Freehling takes occasional moral doubts about slavery as indicating there are lots of would-be abolitionists in the state and how heavily he leans on and features individual biographic sketches. Freeling is mostly high politics, particularly in Prelude, but for an examination of how the South got to be the way it was in terms of working politics and political culture, Ford's Origins of Southern Radicalism is the standard work.