Depends on what your product is and what your market is.
If its commercial grade, shrink-wrap quality and for download, then like all the other "micro-isvs" out there your need to get in front of your potential customers eyeballs.
Step 1: You need a web site, very carefully put together, which sets out what your product does and the features, benefits, and its cost.
Step 2: You need to drum up viewers. You will need to spend money. Pay for some Google ads - set a sensible budget. You can also find potential customers, or classes of customers. Find the web sites or user forums where they already go, and see what you can do to post in there about your product or purchase advertising. Watch out for policies that forbid plugging products in these places, though, you might get banned.
Step 3: Give away some copies, to try and get interest / conversation.
Example: Suppose you had some new photo processing software... then that tells you where you should be directing your initial sales efforts. There are lots of photography forums and web sites.
If you have trouble developing a web site, then there are plenty of people out there who will do it for a fee.
You said in the OP that you have a budget of $0. If you are not prepared to spend anything at all, you will almost certainly fail. You need to spend money to make money.