Web design

Hey, I'm looking to learn web design, but i have no idea what language to learn.

The only prior programming experience i have is the first 100 pages of a C# book.

What language(s) should i learn and what book(s) should i purchase?

Haskell question:

How do I calculate the cosine of 90^o?
cos pi gives -1, which is cool, same with cos 0 => 1, but cos (pi/2) => 6.123031769111886e-17 instead of the expected 0. What units does Haskell use?