There's a few things you'll need to do.
1. Create a software product that people will want to buy. You could create an entirely new product or create a product that serves as an add-on to an existing product. For example an add-on for Quick Books. Via this later route, you can leverage their existing distribution network (Quick Books has a marketplace for example). You could also create a new product and sell it and market it online. Breaking into a retail store is difficult and expensive for a small software vendor, but doable.
2. You'll have to support the product as well, so make sure you have good support in place
3. This is the simple answer, but keep in mind that selling the product and supporting the product are key. Also, ask yourself if you will have one product or have a roadmap for a line of products. What about hosted applications - will you be doing "boxed software" or selling software as an online application?
Editor and Technology Evangelist,
Ramon Ray is editor of SmallBizTechnology.com and author of Technology Solutions for Growing Businesses.