The hypothesis is what you are aiming to prove. You need to persuade your reader that your topic is worth researching and funding. Also, outline how your research will impact the community, the world and the environment, if applicable.
Put as much background information as you think your readers will need to accept your application letter. You want to include the most compelling and persuasive information you can find to get your reader to agree with you. Do not assume that your reader knows everything about your topic, but keep in mind that you are not writing to a toddler.
Your readers will need to know who you are and why you are the right person to carry out this project. List your education, previous projects you worked on or for which you volunteered your services and if any rewards, certificates or recommendations you have received. Lastly, remember to include your contact information and the fact that you intend to follow up with your reader at a later time.