How can I ensure that my code, program, or software is functional and secure?

I can ensure that my program is functional and secure by writing a variety of tests to make sure that my software does what it is supposed to do. Assertions inside of my unit tests makes sure that any code that is pushed that may cause regressions, can be checked against a known set of values. I can also use unit tests to make sure the user cannot put in values that can cause my program to reveal sensitive information or work in an unexpected way.

How do I interpret user needs and incorporate them into a program?

User needs can be distilled into software requirements that act as a basis for my program. These requirements can then be translated into code. Tests can be run to ensure that the code operates as inspected. Finally, user acceptance tests can occur to make sure that the program actually does what the customer wants it to do.

How do I approach designing software?

Software should be designed in a requirements driven fashion. It should also be tested frequently to ensure that bugs do not reach the end user. These tests can be carried out via unit tests, system tests, end to end tests etc. The software should always be progressively developed using continuous integration to ensure that new features can be released quickly, accurately and securely.