The best starting point is at MS MSDN: http://msdn.microsoft.com
This is the online bible for windows APIs and all MS related products. You have to look for the API function 'SetParent', but it might be that you have to set different window styles/properties for you window via 'SetWindowLongA'.
The VB declaration are:
Public Declare Function GetParent Lib "user32" _
(ByVal hwnd As Long) As Long
Public Declare Function SetParent Lib "user32" _
(ByVal hWndChild As Long, ByVal hWndNewParent As Long) As Long
Public Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" _
(ByVal hwnd As Long, ByVal nIndex As Long) As Long
Public Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
(ByVal hwnd As Long, ByVal nIndex As Long, _
ByVal dwNewLong As Long) As Long
You can try google to find some VB related articles about those API functions.
Hope this helps
Thomas