There are a few some things to consider before designing or developing your app.
First get to know the platform in general
if you want to review some basics there are also some C# questions often asked.
You need to design your app before developing. When thinking about designing your app you need to think about three main things. Color, Iconography, and font.
Design for developers:
Now it is time to develop and know what APIs you can play with. With 8.1 Development there are 2 things to keep in mind, your XAML and your Code-Behind.
http://www.microsoftvirtualacademy.com/training-courses/building-apps-for-windows-phone-8-1-jump-start (This is the most important link on this page. It takes you from A to Z )
Now you have to publish your app. Go to the link below and create an account and your app should be published in a few hours. YES, unless there is something absolutely wrong with your app it will pass certification in a few hours.