How to create a "global" variable ?

An easy way is to declare an internal static field or property in Form1. For example:

class Form1 : Form
{    
    internal static MyInt;

    public Form1()
    {
        InitializeComponent();
        MyInt  = 42;
    }

    // etc
}


// code to access this variable from Form2:

int i = Form1.MyInt;