After open any a program webpage, would you please see the subject and the description of the program? You should understand what the program do and how to input data, then input data by prompt, finish click Calculate button to get the result. If the author of the program has made the source code of the program public, you can have a look at source code by clicking Show Source link.
If the author of a program requires payment before you can run program, you should only run the program with default value which the author provided to test the program, if you are satisfied with the program, you should click Purchase button and pay a fee.