The Common App personal essay is your chance to tell admissions readers something that your transcript and activity list can't. It must be between 250 and 650 words, and you choose from a set of seven prompts (including one where you can write about any topic). Prompts are reviewed periodically, so confirm the current list on the Common App site before you start.
Start with a story, not a prompt
Most students begin by picking a prompt and searching for an answer. It usually works better in reverse. List a few moments, challenges, interests, or relationships that have shaped how you think. Then see which prompt fits the strongest one. Almost any good story fits at least one prompt, and the "any topic" option covers the rest.
Show one moment in detail
Essays that try to cover your whole life tend to read like a resume in paragraph form. A single scene, decision, or turning point is easier to write and more memorable to read. Use specific details, such as what you did, what you said, and what you noticed, and then say what you learned and how it changed you. Reflection is what makes it a personal statement rather than just a story.
Write in your own voice
Read your draft aloud. If it doesn't sound like you talking on a good day, rewrite it. Admissions officers read a very large number of essays, and plain, honest writing stands out more than impressive vocabulary does.
Revise with free help
Plan on several drafts, and start early enough to let a draft sit for a few days before you reread it. Ask an English teacher or a trusted adult to read it and tell you where they got bored or confused. Peer swaps with a classmate also work well. Check the draft for length, grammar, and clarity, but keep every idea in your own words. Applications ask you to affirm that the work is yours, so don't let anyone else, or any tool, write it for you.
Don't forget the supplements
Many colleges add their own shorter essays, such as "Why this school?". These deserve real research into specific programs, classes, and opportunities at that college. Generic answers that could apply to any school are easy to spot.
The honest bottom line
A strong essay comes from a real story, specific details, honest reflection, and enough time to revise. None of that requires a paid editor. It requires starting early and being willing to rewrite.