Exactly how depends on the "relationship" between the two forms.
Have a look at these, one of them will fit your circumstances.
The form that creates an instance of another:
MyForm mf = new MyForm();
mf.Show();
Is the "parent", the other form is the "child".
(This doesn't imply any formal MDI relationship)
Transferring information between two forms, Part 1: Parent to Child[
^]
Transferring information between two forms, Part 2: Child to Parent[
^]
Transferring information between two forms, Part 3: Child to Child[
^]
The code is in C#, but it's pretty obvious and exactly the same process in C++.