For mine, I wrote about why I wanted to do Optometry in the first paragraph and how I came to that decision.
The main, middle section should be focusing on any work experience/volunteering you've done. If you've had work experience at an optician's, then that's amazing- talk about anything you learned about the role of an Optometrist and the skills they need. You could also mention how they communicated with their patient, the equipment you saw being used, how the staff worked well under pressure etc (this shows that you have a good knowledge of what working in Optometry is like)
If you haven't had optometry related work experience, then that's okay (especially with the covid situation right now). When I applied it wasn't compulsory to have work experience like medicine/dentistry. You should always mention skills you've gained from other experiences e.g. volunteering, part-time jobs, NCS, DofE, leadership roles in school. A few examples of transferrable skills to Optometry are: communication, logical thinking, accuracy, empathy, problem-solving, organisational. These skills can also come from your A-Levels.
You should also show that you're genuinely interested in the course. Talk about any books/articles/magazines you've read. The New Scientist website has loads of articles about the eye if you're struggling to find resources. Or talk about your interest in a specific subject related to Optom like physics or biology.
The last paragraph should be brief and include your hobbies/interests/awards, anything that makes you stand out
Hope this helps, good luck with your application!