Ooh, this will be fun.

Let's lay some groundwork first.

I do not even know how the war started or who started it.

For most of the nineteenth century prior to the presidential election of 1860, the federal government's ability to limit the spread of slavery had been the central issue of national politics. In 1860, Abraham Lincoln won the presidency on a platform of stopping the spread of slavery into federally-controlled territories - regions that had not yet been organized into states - in the West, despite not even appearing on the ballot in most of the Southern slave states. Fearing that containing slavery would lead to its eventual destruction and furious that they could be ignored in a presidential election, eleven slave states declared their independence from the federal government, organized their own country, and fired on federal soldiers.

All I know is that Gettysburg was the most important battle

The best known, certainly, but not necessarily the most important. General Grant's campaign down the Mississippi, capped by the Siege of Vicksburg, cut the Confederacy in half, starved the large rebel armies east of the Mississippi of food and supplies, and allowed federal troops to march into the rebel heartland from the rear. Sherman's campaign of devastation that razed much of Georgia and South Carolina started from the west.

But the northeners? He was their enemy. Why did they consider him a hero, too?

After the war, white Confederates launched what we would now call a PR campaign, aimed at reconciling with the North on terms that would ignore the rebels' role in starting and sustaining the bloodiest war in American history. Two kinds of common ground were immediately apparent: whiteness, and the heroism and bravery soldiers on both sides showed during the war. Robert E. Lee embodied both of these characteristics. Among the oldest of old money Virginia families, Lee had all the grace and gentility of the South's planter class. He was famously better-dressed than Grant at the surrender negotiations at Appomattox, and had done his best to be gracious to defeated white opponents (while summarily executing captured black soldiers). He was also a reluctant secessionist, staying in the federal army in the early days of secession and defecting only when his home state of Virginia announced its departure.

Northern whites were at first slow to embrace former rebels, but the PR campaign paid off, aided by total dysfunction in federal politics, a sense among veterans that only those that had seen combat could ever fully understand each other, and uncertainty about how to integrate freed slaves into society. As Reconstruction dragged on more than a decade after the war's official end, war-and-tax-weary Northern whites were eager to find some kind of common ground with their Southern cousins. Veneration of Lee and Grant as co-equal national heroes was the most direct way to heal the national divide among whites.

I recommend David Blight, Race and Reunion and Eric Foner, Reconstruction for deeper looks into this fascinating period and subject.