There are many ways that you can pass data between Forms. You should first consider 'Form' is nothing but a class and if you know how to pass data between the classes then that's it, your done.
These are very basic things you should know before you start coding. You should be strong enough with the basics to become a good programmer otherwise you end up with mess.
To answer your question, you can declare a public variable in your form class to set the Combo Box read value. As Chris mentioned in his comment we dont know how your forms are invoked or when it gets displayed. but the following solution may help you to get going with your problem
Assumptions
You want to pass the combo value before the second form get displayed
Form1_Combo_SelectionChange(sender,eventArgs)
{
string selectedText = Convert.ToString(myCombo.SelectedItem);
Form2 secondForm = Form2(selectedText);
secondForm.Show();
}
let me know if you are doing something different.