As Gary Halbert said, Find a starving crowd. So first find your ideal client. Then identify their fears and frustrations. Your product should help in overcoming these in a way your competitors don't. 

You also need to get a pricing strategy in place. That's a whole other subject. 
Finally do a test marketing run and check the response. Tweak your marketing copy and design based on the feedback.
Then scale your marketing using Sales funnels, webinars, paid ads, social media. The list goes on.

This is just a snapshot. 
The devil is in the details.

Would be great to get your insight @Tim Stokes @Farhad Khurshed @Sandy Moore @Tanya Williams 

Thanks @Farhad Khurshed , great insight