Here are a few tips on writing your application essay/personal statement/letter of intent/whatever you want to call it:

• Follow the directions given on the application! The directions tell you what the committee wants to hear, and you would be wise to pay attention to those directions so that you can present yourself as favorably as possible.
• If there is anything about your academic record that would benefit from further explanation (e.g., like why you nearly failed almost all of your biology classes), this essay is the place to provide that explanation. You are allowed to explain yourself and even recompute your GPA in favorable ways in your essay. For example, you might let the committee members know in your essay that if you remove all of your pre-med classes from your transcript (after all, you're not a pre-med major anymore, you might say), then your GPA would be 3.35 rather than 2.93. Of course, be honest in your essay, but within this limit, do everything you can to present yourself favorably.