Basically what you do is you put together your website and your first 2-3 posts will be fairly general, you could do some research on the product creator and just give a basic background about him/her. After you've done that it's usually a good idea to make a post titled something such as "Product Name - Is It A Scam?" this will help you rank for the keyword "product name scam" as often people search for that to check that the product isn't a scam.
Once you've got your site up there are 2 ways you can write a full review:
1) Go to the JV / affiliates page for the product and get as much information as you can, sometimes product owners will detail exactly what the product is on their JV/Affiliate page which is useful, but sometimes they'll just give an overview, you can usually get plenty of details though including banners for your website.
2) Email the product creator, tell them that you have put a review website together and give them the URL, and ask them if they could give you a review copy of the product, also state that if they aren't able to give you a review copy if they could give you an overview of the product / what the product actually is that would help... this way you'll be able to get the info u need!
Next... write your review
Hope this helps,
Mark
PS - What you also should consider is that google can't tell the difference between a normal article on your website and a review article, so you could potentially set up a post / page on your review site called "Product Name Review" and then write a quick summary saying that soon your full review will be there, that way you'll have the page set up with the right title and permalink, all you'll need to do is simply update the content!