Write a program which converts a measurement from one system of units to another. Read a number (or numbers) in one system and output the equivalent number (or numbers) in the other system. Instead of thinking of it as a conversion, it could also be a calculation, eg, to calculate the air pressure from the altitude.
Start with Kilometers to Miles as an example of how a conversion program might be written.
Iterative programming is the process of writing some of the program, compiling and testing it, then writing more, compiling and testing that, etc. Keep going in this cycle until the program is done. The iterative style generally has a much higher success rate than writing the entire program, then compiling and testing it. You may also have to make greater changes to it, eg to read two values, one for feet and one for inches.
Change names. You're welcome to start with the Kilometers to Miles program, but be sure you change all names and comments to reflect the problem you are solving.