Use If / Then statements to build your string:

Code:
If TextBox1.Text.Trim.Length > 0 Then
     strOutput = "Who:" & vbTab & TextBox1.Text & vbCrLf
End if
If TextBox2.Text.Trim.Length > 0 Then
     strOutput = strOutput & "When:" & vbTab & TextBox2.Text & vbCrLf
End if
etc...



Write to the clipboard using:

Code:
Clipboard.SetDataObject(strOutput)