Read over the guidelines for the grant essay carefully. They will often give you more insight into what direction to take. For example, for a need-based grant, you might focus more on financial hardships and how you will use the money to overcome that and achieve your goal. For a service-oriented grant, you would focus on your past experience helping the community and how you will use the grant money to continue to do so.
Develop a thesis statement for your grant essay. The thesis is the primary reason why you merit the grant. All other parts of the essays should be providing details about why you merit the grant and elaborating on those details.
Break away from the pack. Talk about things in your grant essay that separate you from others who are also applying for the grant. Specific experiences you've had, hardships you've faced or honors you've been given are all ways to differentiate yourself.
Include anecdotes in your grant essay. Since the topic is "Why I merit a grant," you can feel free to talk about yourself on a personal level. For example, instead of saying your family doesn't have money to give you, talk about the specific circumstances that led to your family's financial hardship and how those circumstances made you feel.
Add emotion to your "Why I merit a grant" essay. Again, the topic implies that the essay can be personal. This means you can emphasize your feelings and emotions toward different circumstances and causes. The people awarding the grant will be able to identify with you, which makes you a more attractive candidate.