Edit the form constructor. If the form1 is the one you open first, leave it like it is and edit the constructor of the second one so it accepts a form instance as an argument, like this:
'this in your second form
Dim firstform As Form1 'variable to hold an instance of the first
'form that will be available to the whole class
Public Sub New(ByVal frm As Form1) 'accept an instance of the form
MyBase.New()
InitializeComponent()
firstform = frm 'assign the passed in instance to the variable that can
'be accessed in whole class
End Sub
Then you just access form with firstform.TextBox1.Text = "something" or however you want to call it.
:)