I am trying to write an essay on the cause of the fall of the Roman Republic. I have my general outline:
Opening: The primary cause of the fall of the Roman Republic was growing instability in its political, legal and social systems as the republic grew in territory and population.
Point 1: Political (governing) instability caused by the Inability of Roman Republic's political system to adapt to the rapid growth/expansion of lands and people
Point 2: Legal (laws) instability caused by the Inability of Roman Republic's legal system to adapt to the rapid growth/expansion of lands and people
Point 3. Social instability between classes caused by disagreements between these classes about legal, political and military matters.
Point 4: This instability created in the people a frustration with the ineffective Republic form of government and a growing desire for a single leader to come in and save the day.
How do I make this into an actual essay. Especially, how do I turn my opening sentence into a paragraph. Also, what should I put in my closing paragraph?
There are two different areas you might work on to build this into an essay. The first is using specific evidence to support your fairly general claims. In your introductory paragraph, you might want to define what you mean by "instability". Although you use the term multiple times, you don't really define it or give examples. You might want to pinpoint issues like slave revolts, riots, and armed conflicts between different factions.
When you talk about the Republic adapting to control an expanding territory, you should discuss in some degree the actual forms of provincial administration, and it potential for abuse. Cicero is a good source for this. You also might want to address the issue of the governance of Egypt and questions about the stability of the grain supply.
On your fourth point, you might want to talk more about the conflicts between the Senate and other classes, and the popular support for what you call "strong leaders" as plebs supporting individual tyrants who offered them "bread and circuses" as opposed to the Senatorial class which wanted a more oligarchic locus of control.
In your conclusion, you might want to expand on whether the fall of the Republic was really inevitable or whether it was due to accidents of factional politics.