How to start using C# and Javascript/HTML/CSS all together?

I have recently returned to coding after several years away from it. Specifically, I am working towards getting an entry level testing internship with a software development company that I know uses C# and Javascript/HTML/CSS in their work. I have started by getting more familiar with C#, as my previous experiences have been with Java an the two are exceedingly similar for my skill level.

I want to start working soon on building my own web apps as experimental test runs, and am curious what I should do to start integrating all of these languages together into a single program for a cohesive experience of how they work with one another.

Essentially, I was hoping that someone could briefly enlighten me on where to start with this process. I realize this is perhaps a broad question, but it is vague to the point that searches haven't turned up much so far. Even a link to more information would be very helpful.

Thanks!