I'm running a problem with install the junit to the computer. I already download the junit to my computer. But how do I install it? I try to google it, but I don't understand their instructions. I remember in the class, my teacher do a lot of copy and paste stuff from junit folder to the java folder. But I don't remember how to do that. I'm using windows 7 and eclipse IDE. How do I install the junit to my computer first before I set up in eclipse? Thank you so much.

Tell your teacher that what he is doing is wrong and he is giving you bad habit. Any IDE has an option to add library(JAR) to project. Here is how-to for Eclipse.
Other ways you may learn later includes dependencies management with Ant, Maven, Ivy or other tools.
In any case you should never mess with Java installation directory, beside giving system path to it.

Thanks. That help a lot. Thank you so much.