Web Developer Knowledge

My name is Cristian, and I am currently a Junior on Devry University. I am studying CIS - Web Developing. Since I am only in the process of becoming a Web Developer, I would like to know several things about what I should know to become a very experinced one.

First, I would like to know, since I am going to be creating a code for a website, if I need to understand CIS Web Security so that I am able to do my job correctly on a daily basis. Second, what are some web developing languages I should learn in order to build a very nice-looking and creative webiste? Are there any languages that should be prioritized and learn first than the other ones? (I have beginner knowledge on HTML and CSS languages).

Thank you for the answers you provide.