A strong personal statement should be clear, succinct, easy to read.
It should be discuss the relevance of your further reading and experiences - eg if you've visited a massive Dam, the Burj Khalifa, Golden Gate Bridge, places such as the Palm Islands or Hong Kong (the construction involved reclaiming the oceans and fighting natural forces), or its inverse with examples of poor Civil Engineering and how its failures impacts local communities and the world at large like the Hyatt Regency collapse, Banqiao collapse, the Fukushima incident and how they've changed policies around the world, etc - and how those experiences led you to the position of applying to the course.
You should demonstrate your passions by discussing/applying your extracurricular knowledge from all sources, whether it's reading "Structures: Or Why Things Don't Fall Down" or publications from the American Society of Civil Engineers or elsewhere. And don't forget, your role in the future will be cross-disciplinary so the challenges of related Engineering disciplines will be relevant to you too, like developing cost-effective graphene to strengthen existing material or applications of transparent solar panels.
It should discuss your academic strengths, whether you had any leadership qualities at school, the field trips you've attended, the projects and practical assignments you've done and their relevance to the course. If you've studied mathematics, further mathematics, physics, chemistry, and any mechanics modules then you're in luck because those have the most direct relevance - chemistry is about how matter interacts, and physics is about mathematical relationships used to model the world.
Discuss how they've helped you develop your analytical skills, mathematical/scientific reasoning skills, and how they provide a foundation for your Higher Education studies. If you've studied a literature-based subject on top of two sciences, then discuss how it strengthens your teamwork skills or ability to work in a group as well as any (oral/written) communication skills.
And lastly, depending on where you're applying to, discuss a little beyond the scope of your main interests - this should be about a single paragraph. Do you participate in interesting societies, or are involved in something that other people will find really fascinating? When it comes down to it, the last few places will be given to applicants who demonstrate their life is beyond academics alone and have participated in something unique and memorable.
Try to avoid making a generic personal statement, or using too many personal statement clichés like droning on about how you've
always been interested in a subject.
A good rule of thumb when applying to Universities is to apply for one optimistic place (one grade above predicted), two/three realistic places (requirements at predicted grades), and one/two insurance place(s) that you're almost guaranteed to be accepted on no matter what happens (requirements two to three grades below predicted).
Some videos you might like to watch
https://www.youtube.com/watch?v=DvqFG-C9RvYhttps://www.youtube.com/watch?v=5jQ3MJgdkJYhttps://www.youtube.com/watch?v=JWxFVtSUAsQ