Everyone has given great suggestions! I, too, recommend that you join the mentor program if you are nervous about your ability. I had been an advance reviewer for about 6 months before the mentor program was available, but still decided to join it so I could hone my skills. It takes time to be as good as some of the reviewers are, and trust me, most of them didn't start out writing perfect reviews. In time, you will find what style of writing suits you, and what types of things are absolutely necessary to include in a review.
Like Dwtim said, if you don't want to do the mentor program,I'm sure someone would be happy to proof read and offer some help. Feel free to message me if you have any questions. I may not be the highest ranked reviewer by any means, but I'm sure I could offer you some help- even if it means referring you to another reviewer for advice.
But, all in all, the best route is to join the mentor program, get to know the community, read reviews and make note of what people consider extremely useful. In no time, you'll get the swing of things and feel like you've been reviewing forever!