我如何理解循环以及如何使它们正常工作

I am trying to understand how loops work but I can't seem to piece it together. I am working on a shipping java program and whatever I try will not work. So far I have when the integers are correct it proceeds with the program, but I want it when I put in an invalid number (such as zero or a negative) to ask again.I am trying to understand how loops work but I