The first thing I can suggest is to read reviews for things similar to the item you're about to review. If you read a few poor reviews, what questions are you left asking yourself? Make sure you include that information when you write.

Different writers have different styles, some review products very factually, and others tell stories as they write (as though they were describing the product to their friend). Write in the way that suits you best, but be sure to include both nuts-and-bolts info about your product and your own personal thoughts about it.

If you want someone to help you out with your first ~2 reviews, take a look at the mentor program. There are lots of friendly, helpful mentors. Splendwhore helped me a lot. Look at the list of mentors available and read some of their reviews. When you find one whose style is similar to yours, send them a message asking if they'd like a new student. I'm sure they'll be glad to hear from you.

And, most of all, have fun!