Research your topic to generate ideas during prewriting. Search the Internet or library for articles and books on your subject, talk to people who have expertise in the subject or discuss your topic with friends and family. Take notes as you read or during any interviews you do. Research can help you develop an opinion on your topic and provide you with interesting facts and details you can use when you draft your essay.
A variety of writing activities can help you think of more ideas for your essay. Freewriting -- where you write as quickly as you can for five to ten minutes without worrying about spelling, punctuation or whether your ideas are good enough -- can help you feel less self-conscious and break through a mental block. Ask who?, what?, when?, why?, and where? of your topic to think of questions your reader may want answered and help you identify areas you should research. Make a list of words, phrases or examples on your topic. If your essay is about why you want to attend college, think of all the reasons you have for furthering your education.
Prewriting can help you choose your approach for the essay. Decide if you want to narrate a personal experience, describe something so it comes alive to the reader, define a term, persuade a reader of your argument or compare or contrast two subjects. Identify your audience. Your word choice and the information you include will depend on your reader's age and level of experience with the topic. Select your thesis: the main idea your essay will prove or explain.
Organizing the ideas you generate during prewriting can help you prepare to start writing and see if you have gaps that need further research or brainstorming. Look through all the ideas you have developed so far to find connections and similarities. Create a tree diagram of your subject by writing a word or phrase describing it at the top of a piece of paper, listing all the possible subtopics you can think of below that word or phrase and listing all the details and examples you can think of within each subtopic. Outline your essay, making sure to state your thesis, supporting points and the examples you plan to use within those supporting points. Toss out ideas you generated that no longer seem relevant to your essay.