This is a very, very broad question that literally dozens of books have been written about over the past 150 years.

There has been a couple of questions regarding the ACW in this fashion that have been included in the FAQ however, including this one and this one. It could be a good idea to start there, though of course, this doesn't preclude people from answering your question in this particular thread.

The ACW was fought over slavery: on one hand, the existing slave states wanted to expand 'our peculiar institution' into the territories in the Midwest; on the other hand the Southern states feared that the North and the strong (comparatively) abolitionist movement there was working at outlawing slavery even where it already existed. The claim of 'states rights' being at issue isn't wrong per se, but is used to muddle the waters more often than not to avoid the bondage of man by man.

It started when Lincoln was elected President, the Surgeon states began seceding (and formed the Confederate States of America), CSA forces laid siege to several federal forts in their territory, and shots were fired on Fort Sumter.

This is an incredibly superficial answer, and I recommend reading a book or ten. Battlecry for Freedom by McPherson (no relation to the general) provides a very good, thorough introduction to the topic.