If the extension is still in development, then david63's suggestion of posting in the Extensions in Development forum is the best course of action.
if you've completed the extension, you will need to go to the Extensions Database
and click the "New Contribution" button at the top left. You will fill out a multi-part form and it will ultimately submit the extension for review. The Extensions Team will look over the code and do some testing and determine whether or not the extension is ready to be released into the official database, or if they need you to make some changes for security or bug-fixes. Note that the review process can take a while, so be patient.
If you want user feedback while you wait for review, you can still post the extension in the forum we mentioned first.