If your instructor has given you a task description, this would be the place to start the process. I think ensuring that you are clear on all of the components of a task description is going to be critical. Perhaps, opening a dialogue with the instructor might be a good start here, if that is possible. The next step would be, as previously suggested, to begin assembling what you have and what you need in order to compose a strong essay. This would be where going back to the task description would be very helpful. What do you need to contain in this writing that has to be proven? Being able to organize your findings along these lines would help tremendously. I would suggest to you that keeping track of all your sources is going to be very important. Seeing that you are starting the wide ranging process of assembling a paper, there is a tendency to lose sight of the little things such as sources and citations. This will be tougher to do at the end when you have go back and try to scrounge everything back up than if you were able to keep everything in one area or location so that when you finish writing this (and, don't worry, you will), you can go back and source and cite everything properly without panic or fear.
Like any essay, after you've gathered your research materials or ideas, two things are most important: organization and style. I'm confident you can write this biography with both.
Organization is the first key to any successful essay (and most other writing, as well). So, look at the information you've gathered and decide how best to present it. For a biography, something chronilogical might be best--moving from the beginning of his life to the end. Another idea is to divide his life into sections, such as his life, his career, and whatever else is significant about him. (I must confess I only know him for writing "The Most Dangerous Game," so I'm not much help with what your categories might be--but you've done the work and can figure them out, I know.) Really, whatever seems to make sense--generally three sections for a normal-sized essay, but that's also your choice.
Style is a word which covers many things. In general, what I mean by style is writing in a way that makes what you have to tell us interesting and believable. Your readers are more enticed--as you would be--if your language is lively and effective. It's important to say what must be said; but it's equally important to say it effrectively. Vary your sentence structures (make sure they aren't similar in length and form) and use language designed to capture and maintain interest.
Then, of course, pay attention to transitions, documentation (if applicable), grammar, and mechanics. They matter.
When you're finished writing, check out the enotes site below to get some more specific or directed help. Happy writing!