how do I get the value of a different class

Usually a class doesn't get a values from another class. On the other hand, an object, instance of a class could get a value from another object, instance of a different class.
That could happen in several ways. The most evident uses a public 'accessor':

class A
{
  int i;
  public A(int k){i = k;}
  public int the_i
  {
    get { return i; }
  }
}
class B
{
  int j;
  public B(int k){ j = k;}
  public B(A a){ j = a.the_i;} 
}