Start With Security
If the app you're building matters to the company in any way, then it's almost certain to involve sensitive information or information that is private to the user. In either case, you need to keep that information secure. You should start the process of thinking about security at the beginning of the development process, not the end.
Thinking about security from the start means knowing what kind of input you'll be dealing with so you can build data testing into the app. It means understanding the network components and communications mechanisms so you can secure the data in transit. Security means knowing what authentication mechanisms are in place and what ones are available for use, so you make sure only authorized users get involved with data. It means a lot more, too.
Ultimately, though, it means building security into the app rather than bolting it on at the end, and that's the critical difference. So make sure that the very first whiteboard at the start of the project has "security" on it -- your users and your organization will be grateful you did.
(Image: geralt via Pixabay)