C#, Java 5 ... not a whole lot of difference, really. The two languages are even related and share a common development history.

The "Head First" O'Reilly books will give you a good, quick start.

* Head First Java (you can probably skim a lot, knowing OO and C#)
* Head First Servlets and JSP
* Head First Design Patterns

Go download Eclipse and give it all a shot.