I get a lot of questions about the when/how/why of applying to Graduate School. Here are some preliminary thoughts:
1. Why do you want to go to graduate school?
Take some time to think about this question. If the answer is "I don't know what else to do," "I have no marketable skills," or "I want someone to give me step by step instructions on what to do each step of the way" then I think you should reconsider. The first answer implies a lack of self-knowledge which is completely natural at the age most people are graduating from college. I'd recommend joining the Peace Corps or entering the job market first. Students who go directly to graduate school from their BAs (as I did) frequently flounder as they try to determine their interests. Plus, having "real world experience" to supplement the theories that graduate school focuses on improves one's own experience and makes a contribution to the classroom.
The second answer, "I have no marketable skills" is similar to the first. In most cases, you do not need to add more content/experiences/education, you need to learn to market yourself. Spend some time crafting a resume and an online personality. Build a website. Brand thyself. If you need help learning how to talk about yourself, I suggest using the book The Perfect Interview. It contains a number of exercises to learn how to speak about yourself, and to market your unique skills and experiences.
The third answer, "I want someone to give me step by step instructions" is a natural response to the many choices that young people face. Students have very few choices in high school on what classes to take. Their choices expand somewhat at the undergraduate level, but they are still given a rubric by the college of requirements. After the BA, students are frequently overwhelmed by the options they face. This is natural, and not at all a sign that one needs to go to graduate school. One needs to develop preferences, and this is better done by following the advice given above: get a job, join the Peace Corps or Americorps, read the newspaper. When you start getting really angry about something, that will probably illuminate your future.
2. What skills do you want to develop in graduate school? How will you market them?
If you've decided that you need to develop a skill or knowledge base in order to pursue your career goals, then it makes sense to go to graduate school. This is particularly true when you want to perfect language skills or develop a skill such as statistical analysis, GIS, or develop analytical writing skills. The real issue, similar to the above suggestions, is that you have to be able to demonstrate these skills (or market them) upon graduation. This requires products. You need evidence. How will you demonstrate that you have acquired these skills? Begin thinking about this process as you apply for graduate school. It should guide your choice of institution and program.
3. When should I go to graduate school?
Suppose you've chosen that you want to develop Arabic language skills. Then there are obvious institutions that would be ideal to attend. One must be honest with oneself in assessing whether or not one is going to be competitive in the admissions process for these top institutions. If not, then STOP APPLYING FOR GRADUATE SCHOOL and start seeking out opportunities that will make one's application more competitive. THERE IS NO RUSH TO GO TO GRADUATE SCHOOL. The worst choice you could make is to go into debt pursuing a degree you're not sure you even want or need simply because you didn't do your homework. SLOW DOWN, think about what you want, and think about how you are going to get there. Most students would benefit from getting a job, studying language abroad, or doing a fellowship of some sort BEFORE applying so that their application will be more competitive.
4. How do I write a Statement of Interest?
I have never been on a graduate admissions committee, so I am not qualified to answer this question. However, after ten years of higher education, this is my sense: your statement of interest needs to communicate how the graduate program will get you what you want at the same time that you communicate to the graduate school that IT WILL GET WHAT IT WANTS. What are you bringing to the table? How will you make them look good? Did you get outside funding? Have you had past fellowships that look prestigious? Do you bring interesting work experience to the table? If you cannot write this essay, then think about the sorts of things that you wish you could write in it, and then go out and do them! Get a job! Live abroad! Become deeply involved with a charity that is doing meaningful work!
Disclaimer: This post may make it seem that I am hostile to graduate school. I am not. But I do think some people use graduate school to postpone decisions that they really just need to make. Being an adult means making choices, often in situations of imperfect information. Its ok to change one's mind later, but in the moment, I think the best thing that someone applying for graduate school can do is MAKE SOME DECISIONS. Who are you? What do you want? What can you offer a graduate program? What can it offer you? Until you know the answers to these questions, you haven't done your homework. Stop comparing yourself with your neighbors and start doing some honest self assessment.