How to determine the order of Junit test classes?

This is the package and class structure of the program

com.www.xx
  Class_A.java
com.www.xx.data
  Class_B.java
  Class_C.java
com.www.xx.xml
  Class_D.java
  Class_E.java
  Class_F.java

I am new to Unit testing and I just learnt Junit. I have to write some test cases for a project that was done by someone else.

That project includes 3 packages. Those are

com.www.xx
com.www.xx.data
com.www.xx.xml

Class_A,Class_D,Class_E,Class_F depends on Class_B & Class_C

Example : in the Class_A following imports are exists.

import com.www.xx.data.Class_B
import com.www.xx.data.Class_C
import com.www.xx.xml.Class_E
import com.www.xx.xml.Class_F

First do i need to test Class_B and Class_C because All class of package "com.www.xx" and package "com.www.xx.xml" depends on classses of package "com.www.xx.data"

Please give me some instructions about order of classes that I need to write test cases.