where to post ?

I wanna to post my service of audio to text conversion / transcription.
which forum is good ?

If it is a paid service then you need to open a BST in the marketplace but you will need to upgrade to JrVIP first.
You have been here over a year so should be familiar with this by now.

BHW Marketplace rules and how to post http://www.blackhatworld.com/blackhat-seo/f203-bhw-marketplace-rules-how-post/
Forum Rules http://www.blackhatworld.com/blackhat-seo/misc.php?do=vsarules&cfrset=1