I am anti slavery

what are the differences in the 2 sides that fought in the civil war?
I mean, was one side anti slavery and the other not?

Because the question asks "if you were to fight in the civil war, for for which side would you fight and why"?

Please help...

The Southerners were the people who owned slaves prior to the American Civil War. Slaves were not held in the Northern states.

This is a very simplistic view of the civil war, to say that slavery was the main issue. IT became the issue after the Emancipation proclamation in 1863. But the war started over disagreements over Federal vs State authority (Yankess called the war "The War of the Rebellion"). The North was losing the war in a major way the first two years, there were major anti-war riots in large Northern cities, esp NYC. Now as a separate movement, the abolitionist movement was gaining ground, and inflaming passions. Abe Lincoln took note of it, and issued the Emancipation Proclamitation, bringing that issue to bear on the war effort, in fact, in my opinion to get some support for the failing war effort. It worked, the war now had a moral justification, and war support in the North increased.

So to your question: Which side would you have entered on: Federalism, or the side of States being a confederation? In the end, would you have changed your mind?
You may want to look that the controversy of the book Uncle Tom's Cabin and its effect on the war, and its effect on President Lincoln.
In the end, the Yankees won, for better or worse. Slavery would have went away, in my opinion, even without the war, due to economic reasons mainly, but and partially due to the rising abolitionist movement coming from the Wesley movement sweeping the South. My family had a few slaves, which were freed at the end of the war, due to the war. They went from being a part of the family to economic hardship being cut off. Such happens when hard times come, and harder feelings split families. It is a shame. From what I can read in family history (little is available), folks then wore chips on their shoulder for about three generations, and in fact in politics, may still have very firm beliefs about the nature of things.
So when you write about what side you would fight on, we tend to look at things as we want to see the issues, which may or may not be as it was; and we see things being simple issues of right and wrong, when in fact there were other issues that were the causes that fueled the War.
War is not a nice thing to choose which side to fight, as a lot of innocents suffer, and are bloody for years.
Think this out carefully. Have fun with your essay.

Slavery really wasn't the reason for the Civil War. The primary concern was "states' rights". Did the federal government have supremacy over the affairs of individual states and if so, on what questions and how much.

Here are some sites that be explain more completely.

http://civilwar.bluegrass.net/secessioncrisis/statesrights.html
http://www.civilwarhome.com/statesrights.htm
http://en.wikipedia.org/wiki/States'_rights
http://www.infoplease.com/ce6/history/A0846562.html

During the American Civil War, which took place from 1861 to 1865, there were two main sides: the Union, also known as the North, and the Confederacy, also known as the South. The question of slavery played a significant role in the division between the two sides.

The Union was comprised of states that remained loyal to the federal government and fought for its preservation. The primary goal of the Union was to restore the United States and maintain its unity. While there were various perspectives among Union supporters regarding abolition and the rights of African Americans, by the end of the war, President Abraham Lincoln and the Union government sought to abolish slavery.

On the other hand, the Confederacy was formed by states that seceded from the Union and aimed to establish a separate nation. The Confederacy was fighting, in part, to preserve the institution of slavery, which was a fundamental pillar of their economy and way of life. The Confederate government explicitly stated that slavery was one of the main causes of their secession and their fight for independence.

If you were to choose a side to support in the Civil War, it depends on your personal beliefs and values. If you are anti-slavery, it is likely that you would align with the Union, as they eventually made emancipation a goal of the war. However, it's essential to note that the situation during the Civil War was complex, and each side had various motivations and perspectives among their supporters. It's advisable to study and understand the broader historical context, as well as the specific beliefs and values of both sides, before forming a definitive opinion.