I need to write an english essay (450 words) on "Life" and have no idea on where to even start. Does anyone have any ideas for me?I want to try a...

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.