Forums, like social media, are not all about self promotion. They're there for debate, discussion and help - a sharing of knowledge so that we can all benefit - through which you build reputation.
Rather than just jump in trying to sell your product, try engaging first. By viewing and participating in posts, adding your help or advice where you think it will benefit, you will start to develop an understanding of how things work; and those around you will start to get an understanding of you.
Do it more and more (even better as a paid member, with links to your site), you'll find some people may even take a look and have an interest in what you're selling without you having to say a word.
Just remember, it's not all self, self, self.