Writing a statement of purpose for a graduate program, teaching position, research post, or other academic situation can be one of the hardest writing tasks you'll face. You've got to say a LOT in a short space and there's a lot riding on the outcome. Probably the toughest part is just choosing what to write about and getting started.
Here's a five-paragraph outline to get you started. Don't worry about making it pretty, just get it down. You can go back and polish it later .
Paragraph #1: Who I am
The people reading these things are reading a lot of them, so keep that in mind as you introduce yourself. The conventional wisdom is that they can remember three things about you, so choose them wisely. Start with a story to help them see a mental picture of you in action. This story should be relevant to the discipline and will lead you to a conclusion that sounds something like (but is less cliche-ish than) "and at that moment I knew I would go into...."
Paragraph #2: Where I have been
This is your opportunity to highlight education, experience, and skills that make you stand out as an applicant. You can pick your biggest moment from your resume or CV and unpack it here. Think CONCRETE and QUANTIFIABLE. Don't just say that a particular experience improved your leadership skills, say that as an editor of a university publication you oversaw six section editors, collected 20+ submissions per week, and met daily with the production team. Show, as they say in fiction writing, don't tell. You can start this paragraph with "during my time at X University or X [job]."
Paragraph #3: Where you are going
You have chosen your next steps--career training, professional program, academic or research position--because what you talked about in your two previous paragraphs has led you here. Explain, specifically, why you want to study constitutional law, family medicine, Shakespearean dramaturgy, etc. Why are you the right person for their program? What has prepared you for this next step. This may include references to your experiences, education, and skills that are similar to Paragraph #2.
Paragraph #4: Alignment of values
If you are applying to a specific program or school, these will be the values stated in its mission statement. If you are applying to a general pool of programs with a single essay that goes to all of them, the values will be those associated with the discipline or profession (you can often find these on the websites of professional organizations). This paragraph will say something like "My experience on the programming board of the undergraduate research conference gave me a unique appreciation for the value of collaborative community [extra credit if you can quote those values they list here]." You might say something like "As a former EMT, I share the PA profession's application of the value of human connection."
Paragraph #5: What I have to contribute
A lot of applicants focus on what they WANT from a program, position, or institution. Instead, paint yourself as a GIVER. What do you have to contribute to the program, organization, profession, and greater community? These are people whose lives and livelihoods are dedicated, to exactly the discipline you're hoping to enter, so they're believers in its value. Go ahead and go big. You're asking them to invest in you. What will the return on that investment be? "As a professor of English Literature, I hope to build upon the important work of introducing undergraduate students to the poetry of Indigenous writers...."
Once you've got all this down, you'll have something to mold and shape and polish into an essay that presents you and a strong candidate for their consideration.