First of all, make sure you understand your assignment. 
This seems pretty simple, right? But it's important to take the time to carefully read over what you have been asked to do. Look for words such as discuss, analyze, evaluate and compare. Have you been asked to use specific types of information? How long should your paper be? What writing style are you expected to use?
If something isn't clear to you, ask your instructor.
And...you've heard this before...plan ahead and start well before the paper is due!