As it says in our rules: We're not here to do your homework for you. You may want to go do your own research and come back with specific questions. More information in this meta thread. You could also try /r/HomeworkHelp.

Pick something that really interests you. I know that's a lame answer, but the more natural interest you have in something you'll be banging your head against for countless hours, the better it will be for you.

I did a 20 page term paper on the fitna, but didn't use many primary sources (a translation of Al-Tabari was the closest I got). If you're interested in modern sectarian violence, primary sources are much easier to find, and the objective truth of what happened in the 600's is far less important than how modern Sunnis and Shia interpret it.