Looking to build a Application that displays real time output like a counter. Where do I start

Looking to build a application that displays real time output using formulas like a "counter". In this situation it would be gallons per minute. My target would be for IOS and android. Any idea's on where to start?

I did some research and found a framework called MathJax not sure if thats where I should start of if I should just try to code it myself. My coding is not very good but I am learning.