Colleges aren't just looking at how well you can write, they are looking at your preference and to get an understanding of who you are as a person. Think about what type of image you want to present to the college. For example, if you are applying to a college as a science major, you want to choose an essay that allows you to show your analytical side. Likewise, if you are applying as a drama major, you want an essay that can show your creative side.
After you choose your topic, you should brainstorm before you start writing. Set a timer for 15 minutes and write down everything that you can think about your essay topic. Some people find it helpful to use mind mapping for their brainstorms. To do this, write the main topic in the center of your paper, then draw lines out for different subtopics. Each subtopic can then have more lines for additional ideas. If you're stuck for ideas, try switching to a different colored pen. This can prompt your brain to come up with more ideas. If you have the time, try brainstorming two different essay topics. You may be surprised by how one fleshes out.
Your brainstorming give you an idea of what you are going to write about in your essay. To write it, just sit down and write out your thoughts as they come. Don't worry about how it sounds. You will edit it later. It's important to get everything out on paper. Most essays call for you to write a story. Try to write using a basic three-act story structure. This adds interest to your essay, which will help convince the admissions department that you are a good candidate.
Choose a two or three trusted individuals to review your essay and offer suggestions. Don't ask too many people for advice because your essay can sound too fragmented if you take everyone's advice. Double- and triple-check for grammar and spelling mistakes. Writing "their" when you mean "they're" will make it seem that you rushed through the piece. Remember that even though the spell and grammar check on your word processing software will catch many mistakes, some can still slip through. Reading your essay out loud can help you to catch mistakes.