For citation, www.easybib.com is good.
For sources, I recommend searching for the death penalty on wikipedia, then looking at their source list at the bottom of the page. Maybe also look for statistics (what crimes earned the death penalty, what evidence was presented, etc.)
For writing the essay, you'll need a strong intro and conclusion. I usually write the body first however, and then the intro and conclusion. For the body talk about the history of the death penalty (pre-American too might be good), what crimes warrant a death penalty, a specific death penalty or two, arguements of other people (properly quoted and only briefly) for and against the death penalty. Present facts then start persuading the reader why the death penalty should be retracted.
Hope that helped some. :)