Okay so I'm a noob at this but I'll do my best to explain.
2 things before we start, First there was already a huge explanation http://lejos.sourceforge.net/forum/viewtopic.php?t=355
and also check out the read me that comes with the linux version of lejos
Okay so to start, we need the JDK. Open up the terminal and type;
sudo apt-get install sun-java6-jdk
Second grab lejos off the website and put the folder in a place where its not going to move again
Third we need libUSB to communicate from computer to nxt
sudo apt-get install libusb-dev
Fourth, we need to make or change system variables
They talk plenty about that in the other topic I posted at the top
but for example this is what I typed for my java home path
export makes it a variable from what I was told while the variable name is JAVA_HOME and the path it points to is my java 6 main folder
Then after all this has been done, change directory in the terminal to your build folder in lejos, and type in ant
If your lucky, you will get a message saying build successful. otherwise were still here to help with whatever problems you may have, once again check out the other page I put at the top.
Hope that at least gets you started, If you dont know how to do alot of the stuff i said, you can do more research on google about them, check out the other topics on here, or hope someone else will answer it better for you. I hope i did a pretty good job though and if it looks complicated, dont worry we all have to start learning from somewhere, I was very confused at first too