i installed java and tryed to run a program and i get

java.lang.NoClassDefFoundError

i didn't know if it was the program or if i didn't set somthing up right

The error means
your JVM can't find your java program. Make sure you open the terminal in the same directory as your java class file.

(Alternatively you can give the classpath to it but don't think about it first. Try the first one first)

ok i did what you said and i get the error

#java jitac-0.2.0.jar

Exception in thread "main" java.lang.NoClassDefFoundError: jitac-0.2.0.jar
at 0x40268e17: java.lang.Throwable.Throwable(java.lang.String) (/usr/lib/./libgcj.so.3)
at 0x4025bc8e: java.lang.Error.Error(java.lang.String) (/usr/lib/./libgcj.so.3)
at 0x4025d6b6: java.lang.LinkageError.LinkageError(java.lang.String) (/usr/lib/./libgcj.so.3)
at 0x4025eb36: java.lang.NoClassDefFoundError.NoClassDefFoundError(java.lang.String) (/usr/lib/./libgcj.so.3)
at 0x402ad075: gnu.gcj.runtime.FirstThread.run() (/usr/lib/./libgcj.so.3)
at 0x4024fc4c: _Jv_ThreadRun(java.lang.Thread) (/usr/lib/./libgcj.so.3)
at 0x4021c8ac: _Jv_RunMain(java.lang.Class, byte const, int, byte const, boolean) (/usr/lib/./libgcj.so.3)
at 0x08048910: ?? (??:0)
at 0x406978c7: __libc_start_main (/lib/libc.so.6)
at 0x080486c1: ?? (??:0)

you cannot run ".jar" files like that.

You have to do this for jar files.

java -jar jitac-0.2.0.jar

or:

java -cp jitac-0.2.0.jar [classname in jar, like com.bunk.foo.FooBar]