Apps in the market have a lot of comments and reviews. How do I add my own rating and comment to these?
Once you've actually installed an application, open it up in the Market and the "My Review" section will appear -- you can give it a 1-5 star rating, and then submit comments.
If you haven't installed (or have uninstalled the app)s, you can only view comments (or mark as spam).
See Also: Market comment posting policy.