Total beginner in java

[Andrea Giusto <giusto@economia.unibo.it>]

Hi, 
I am no expert at all in java, but I have some knowledge about c++ and 
programming in general. Since I like to have everything fine on my 
computer ;> I would like to be able to use java applets and programs...How do 
I start it? is there any good comprehensive source of info in the internet 
( perhaps an HowTo?) that deals with linux and java?
And, btw, I downloaded a seemingly interesting program that come with no 
documentation and is a .jar file! Given that I installed Blackdown according 
to the Debian java policy what am I supposed to do with this file?