This really comes down to adequately identifying your target audience, and crafting your PRODUCT/SERVICES to precisely meet their needs. If you are developing apps that you think are good ideas, but you have no market research on which to base that conclusion, that can be a problem. Generally if you start with sufficient understanding of what your customers want/need, then your apps will be so "spot on" they will basically sell themselves.
Of course you may need sales copy, squeeze pages, email follow-ups, or other content but you can learn that stuff when you are selling and making money. Here is a general paradigm regarding sales:
Need + Benefits = Value
When creating sales messages it is important to frame the features of your software in a way that highlights the benefit of the product/service to your target market. Features are what a product/service IS - Benefits are what it DOES. When creating sales messages, I tend to put myself in my customer's shoes as it allows me to have a greater sense for what kinds of things might move me to take action and buy something.
Look around the forum - I'm sure there is a ton of valuable information here pursuant to copywriting, marketing, and sales. Good luck!
