What kind of an essay? Does the mode matter (narrative, descriptive, expository or argumentative)?
Good writing doesn't necessarily "just flow." Contrary to popular belief, good writers are not just born, they take their natural talent and train and practice - like olympic atheletes. Before writing, I encourage you to consider the "rhetorical triangle" before you brainstorm. Who is your target audience? Who are you (what do you wish to communicate as the author)? And, what is your purpose for writing?
To be perfectly candid, as a teacher I try to avoid broad topics like this because it tends to produce whole stacks of "philosophical" ideas. No offense, but philosophy according to 12th graders, in my experience, never quite comes across as original or particularly interesting.
That said, if your options for mode of writing are open, I encourage you to write a narrative essay - and tell a personal story from YOUR LIFE. First, it will be naturally cohesive - less risk of straying from the topic (which, by the way, should be narrowed down from the HUGE idea of "life"). You will also appeal to a much wider audience - it is a fact that narratives are entertaining. As a 12th grader, you can't hope to do much more than entertain, and possibly emotionally move your audience.
I hate to be the voice of reality here, but an adult audience will hesistate to take the voice of a high school student as any sort of authority. The ideas of "biological" and "philosophical" are far less likely to produce something original. A story about yourself, on the other hand, is only yours. Use your perspective and your age to enhance your writing, rather than paint you into a corner.
These are just ideas - to help you get started. Aside from grammar/technical problems, the most prominent problem I experience with student writing is a fear of being authentic and personal - but think about all the great ANYTHING of the world (writers, actors, artists, musicians) - they are often successful because they are just authentically themselves.