Programming and security

Question

so how can you use you're knowledge in web development and programming to find a mistake in a website ? :)
sorry for my bad English

Answer 1

read up about web vulnerabilities and learn to understand them owasp is the best organization to focus on they have great tools and info try webgoat for instancehttps://www.owasp.org/index.php/Category:OWASP_Top_Ten_Project

Answer 2

Checking the codes for exploit...