One historian wrote that the seeds of the American Civil War were planted in the settlements of Jamestown and Plymouth, two and a half centuries earlier. (It might be worthwhile for you to look up that quote...) From the very beginning of English colonization in North America, two different sets of colonists, with different ideologies, religious conviction, social standing, and educational convictions established different colonies, and these, from the very start, separated along the geographical North and South. The economies of both areas were different, and over time manifested Northern industry and Southern agriculture. Even founding the United States in 1776 brought these two divergent areas in conflict (If you haven't already, see the musical "1776" which shows how close the US came to not declaring independence from Britain because of political divisions between North and South) and was only partially resolved by compromise. Throughout the 19th century, this pattern of compromise maintained the US as a single country; when compromise finally failed in the 1860's war was the inevitable result.
Experts have advanced many causes for the American Civil war between the Southern and Northern states. These causes relate to basic difference between their economies and lifestyles. Southern states relied mainly on agriculture, and led a luxurious life. They believed in preserving their past and present culture. In comparison Northern states relied more on trade and industry. They led a hardy life, were more educated, and believed in adopting new ways of life. However, explicit cause of Civil War was the difference between the two groups over the question of slavery.
Though the Civil War started in 1861, the progressive people in Northern states were opposed to slavery since around 1800. Attempts were made to reach a compromise between the people who opposed and supported slavery by passing some laws that put some restrictions on slavery but did not abolish it completely.
As the opposition to slavery grew, Southern states feared that slavery may be abolished completely. When Abraham Lincoln was elected president in 1860 they felt he might do so in near future. To avoid such a possibility, some of them decided to secede from the rest of the country. The first state to do so in December 1860 was South Carolina. Five other states - Mississippi, Florida, Alabama, Georgia and Louisiana - followed in January 1861. In February 1861 they formed a Confederate States of America. Texas joined them in March 1861.
The Confederates fired on Fort Sumter in South Carolina - a Military Post of Union (the undivided USA) on April 12 , 1861. This can be taken as the date of stat of Civil war. The war lasted for four years and ended on April 9, 1965 with surrender of General Lee to General Grant.