Before you start, make sure you have time. Also, it helps if you are an organized person who can keep track of where you got the code for the product, what kind of review you signed up for, how long you have to do the review, and what you have to fill out when you finish your review. Some groups/sites want a form filled out; others simply want your permalink to your review.
Also, do not get involved with too many groups/sites. Be sure to also write your own review; do not copy and paste from others. Do not leave a single line or two; write at least 200 words on any review you do. And, write them well. How many unopened boxes do I have in my living room right now from Amazon? three. Back to work.