Not sure about how to directly answer that - depends quite a bit on your current skill set and what you may be willing to invest beyond time and effort. I.e., purchase development software.
And "compatible with all devices" is pretty broad.
You might start with this article:
http
/www.infoworld.com/article/3054549/application-development/10-simple-tools-for-building-mobile-apps-fast.html#slide1
For more such information google words such as (as I did): "what code to use to write online apps".
The goal is to find a "springboard" that you can start with - may help you select and write code that will be supported by a large number of devices.
That would be the beginning point.