To create an MDI (Multiple Document Interface) application in your Winforms Project, start with creating a new WinForms project. This creates one main form for the application. Go to the Properties window for this form and set the ‘IsMDIContainer’ to TRUE. This sets up the form as an MDI container for child windows.

Being that an MDI form is a container, it is a container for child forms/windows. Child forms/windows are forms that show up only inside the MDI container. To open a child form, you need to add code like the following:
VB:
Dim frm as New Form
frm.MDIParent=Form1 ' Assuming you named your MDI container 'Form1'
frm.Show

C#:
Form frm = new Form();
frm.MDIParent = this;
frm.Show();

Of course, since (more than likely) you will be opening many child windows inside your MDI container, if you are working with VB, you can create a Module with that base code, passing the child window’s name as a parameter (see below). If you’re working in C#, you’ll need to create a class to do it.

Public Sub OpenNewForm(ByVal frm As System.Windows.Forms.Form)
    frm = New Form
    frm.MdiParent = Form1
    frm.Show()
End Sub