First of all, you'll want to know if you are better off writing the content yourself- or if you should find a freelance copywriter or agency to carry out the work for you.
It's a good idea to read a few books on web copywriting so that you can recognise good writing when you see it. This has helped me tremendously, but good books are quite hard to come by, so you may have to read a few to get a balanced view.
Books I would recommend include: Letting Go Of The Words (Ginny Riddish), Write To Sell (Andy Maslen), Persuasive Online Copywriting (Bryan Eisenberg, Jeffrey Eisenberg, Lisa T. Davis).
You will need to find your own style as there are some approaches to copywriting that I'm not comfortable with.
If you want examples of great product page copywriting go to firebox.com - wonderful, exciting, engaging, emotive copy.