This is a huge topic. Entire books have been written on it. I will summarize some of the topics, but to really get a grasp on this you would at least want to read the beginnings of three books:

Paul Preston: The Spanish Civil War: Reaction, Revolution and Revenge

Hugh Thomas: The Spanish Civil War

Anthony Beevor: The Battle for Spain

In brief, the Spanish Civil War had a host of reasons. A number of 20th century movements divided the country, and the nation was not stable enough to contain these movements (not that many 20th century nations were stable enough in the 1930's to contain such movements). On the ideological front, you have a number of movements. Fascism in the form of falangists, who clamored for a right-wing government with a strong leader. On the other end of the scale, you had anarchists and anarcho-syndicalists who were vehemently opposed to such an idea, and wanted to put their ideals into practice. There were also a large number of communists in the country who wanted to put their own spin on things. There were not one but two different monarchist factions, each backing their own claimants to the throne. The military not only wanted to keep left-wing groups from interfering with their power, but wanted to expand their control. Each of these groups was quite willing to use violence to pursue their agenda, and for some of the groups it was an expected result.

Overlapping these political and ideological groups were a number of other issues. There were a number of regional groups that wanted either independence or home rule, most notably the Catalonians and Galicians. Both of these regions had a mixture of leftist groups who were in control, and eventually favored the Republic. In addition, the Basques were desirous of independence, but they were a heavily Catholic region who resisted many leftist ideologies but favored the Republic in order to gain control over their own affairs. Opposed to these regional groups were rabid nationalists, who saw any diminution of the nation's control as an insufferable affront. Going back to religion, the Catholic Church largely supported the Nationalists, but many of her clergy were far too corrupt to sit in judgement of others. While this is a shame, there were a large number of Republicans of all stripes who were actively advocating attacks on clergy--attacks that eventually happened.

I haven't touched on economic instability, or the political failures of the multiple governments that had control over Spain in the post-WWI era. Primo de Rivera's military dictatorship, the abdication of King Alfonso XIII, the 1931 elections that led to leftist control followed by the 1933 elections that swung to the right, and the 1936 leftist election victories that directly preceded the coup all contributed to rising tensions in Spain. But why war? Because a number of generals decided to attempt to overthrow the government, Francisco Franco among them. Through a number of events and manipulations, Franco ended up the head of the Nationalist movement.

Now, the above is a very brief and highly simplified overview of some of the causes of the Spanish Civil War. I left out a lot, and I cannot understate the complexity of this subject. Read a number of books on the subject, particularly Preston, Beevor, and Thomas. Take notes, ask your teacher questions, and come back here to ask even more questions.

As always, followup questions from OP and others are encouraged.

(Edited to add the below comments)

Also, think about narrowing your topic. The complexities involved in the Spanish Civil War are nearly as large as those in WWII. You have many of the same movements, many of the same nations involved, and a ton of compelling stories available in the three years of the war. I would suggest picking something that catches your imagination. If you're a regionalist, looking into the Basques or Catalans could be interesting. Same goes for politics, as picking a political philosophy that you find attractive (or abhorrent) could be fruitful. But the whole war is a whole heck of a lot to chew, so think about taking a smaller bite.