what is the proper way to create an application

I want a simple advice to create an application .I have my idea but I do not know which part of the app I am supposed to start first (Design or code ) I would like your advice in that direction .. it will be easy to finish the app (make it working properly ) with simple design and after work properly on the UI design

or it will be better to make the design and then code accordingly

I would like to get advice and suggestion fact in some situation of code , design and all

thank you