The civil war began because of disagreements between the slaves states and free states after Abraham Lincoln became president in 1860 and he wanted a stronger federal government (not state) and was against slavery
Therefore the southern states didn't want to be a part of the states anymore because they were worried that they would have to outlaw slavery in all states because slaverly was outlawed in the north and they were also concerned that they would lose state power as the US expanded
However the Northern states wanted to stay one country and of course wanted slavery banned
The actual fighting began at Fort Sumter in South Carolina on April 12, 1861 and didnt end until May 9, 1865
It started because Lincoln had won the 1860 election on a ticket of no new slave-states, so the South was doomed to be outvoted in Congress, which would pass laws that favoured the North at the expense of the South. So most of the slave-states broke away to form the Confederate States of America.
As for when it started, there was no actual declaration of war. The Confederacy could claim that it didn't want a war at all; it just wanted to defend its borders. Lincoln could not declare war on the Confederacy, because Congress did not recognise it as a sovereign nation.
The first shots were fired by the Confederates at the US Army garrison on the island of Fort Sumter in Charleston harbour on April 12th 1861, and Lincoln called for volunteer troops. The war was on.
