Well... you have to know HTML and a bit of CSS and JS. On the pascal side, you actually don't require anything. Free Pascal can build server-side application out-of-the-box. But FPC comes with fcl-web or fp-web package that would make everything easier. However, you still can write pascal web app just using "raw" pascal.
Here's my collection of small web app examples made with pascal:
http://beeography.koding.io/viewcode.cgi (It's on a free account, so the server is not running all the time.)
Enjoy!