Do you mean write and sell your own software? or just sell other people's software (afffiliate sales)?
You need to learn internet marketing first. It's a big subject and you will have to learn many things to make good profits through your software product.I'll give you one method possible to continue but I would like to mention you that it's not the only method but it explains the method which is using by majority of well expert internet marketers . So, (strong recommendation) follow internet and improve your knowledge about internet marketing. Method, 1. Create a website for your product. 2. Choose payment processor. 3. Create a splash page (email capturing). 4. Create a free offer. 5. Choose an Autoresponder service. 6. Set a series of follow up emails. Inside these steps there are so many theories, tricks, strategies and gimmicks. Follow internet and good free resources like warriorforum.com and improve your knowledge. Good luck..