How I would write the first paragraph (Feel free to ignore me):
Historians have long debated the main cause of the civil war, and in early years of schooling many are taught that the main cause was due to the outlawing of slavery. This is partially correct, but there were many other causes which culminated in that horrific war. Since there are so many major (And minor) causes of the Civil War, it is impossible to generalize it into one event. This website intends to educate you on every main cause of the war, and allow you decide for yourself what the true causes were.
EDIT: The 'Kansas-Nebraska Act' paragraph was very good. I suspect you copied it

.
As for the Abe Lincoln part, I would have written it:
Another major cause of the civil war was Abraham Lincoln's election; many southerners had problems with this decision, due to the fact they considered him anti-slavery and biased in favor of the northern part of the United States. Even before Lincoln was even elected seven states had decided to break off from the union: South Carolina, Mississippi, Florida, Alabama, Georgia, Louisiana, and Texas. Furthermore, he planned to move the entire African population to Central America, because he believed that "whites and blacks could not live together in harmony." This is one of the major reasons a lot of slaves fought for the Southern side of the Civil War.
A few small changes. I think it makes it appear more professional. I never heard that slaves fought for the Confederacy. Are you sure of your source?