I do it like this:
    Dim Finfo As String
    Dim FilterIndex As Long
    Dim Title As String
    Dim ExportFilename As Variant
    Dim CopyBook As Workbook
    Dim CopySheet As Worksheet
    Dim MnthName As String
    'Set up file filter
    Finfo = "Excel Files (*.xls*),*.xls*"
    'Set filter index to Excel Files by default in case more are added
    FilterIndex = 1
    ' set Caption for dialogue box
    Title = "Select a the DD Revenue Master file to Export to"
    'get the Forecast Filename
    ExportFilename = Application.GetOpenFilename(Finfo, FilterIndex, Title)
    'Handle file Selection
    If ExportFilename = False Then
        'No Export File was Selected
        MsgBox "No file was selected"
    Else
        'Check and see if this is a correct Export File
        Workbooks.Open (ExportFilename)
        Set CopyBook = ActiveWorkbook
        Set CopySheet = CopyBook.Worksheets(1)
        MsgBox "Valid File Selected."
        Application.CutCopyMode = False
        revenueSheet.Range("A1:BO500").Copy
        CopyBook.Worksheets(1).Activate
        CopyBook.Worksheets(1).Range("A1").PasteSpecial Paste:=xlPasteColumnWidths
        CopyBook.Worksheets(1).Range("A1").PasteSpecial Paste:=xlPasteAllUsingSourceTheme, Operation:=xlNone, _
                    SkipBlanks:=False, Transpose:=False
         Application.CutCopyMode = False 'erase the clipboard
'close your stuff that you dont want open
End If
End Sub
Don't forget to close your workbooks when you are done.  I had to trim a bunch of code because my file launches into a large case select.  But often you select a workbook, open it, select some data, copy it, and paste it, close the workbook.  Happens alot.  Hope this helps.  I believe that I found that you had to activate the newly selected workbook to perform actions on it.  You can always refer to the workbook with the code in it as ThisWorkbook
To avoid confusion and since they are used in a bunch of modules I have a global variables module with the following in it but you could do this at the top of the sub if you don't have a complex project.
Option Explicit
Public thisWB As Workbook
Public functionSheet As Worksheet
Public revenueSheet As Worksheet
Public salesSheet As Worksheet
Public scratchSheet As Worksheet
Public lastRow As Double
'**********************************************************
'This sub routine will be used to intialize public variables
'**********************************************************
Private Sub SetPublicVariables()
    Set thisWB = ActiveWorkbook
    Set functionSheet = thisWB.Worksheets("Data Functions")
    Set revenueSheet = thisWB.Worksheets("DD Monthly Revenue")
    Set salesSheet = thisWB.Worksheets("Salespersons")
    Set scratchSheet = thisWB.Worksheets("ScratchSheet")
End Sub
I use this method alot . . . . . . 
Oh, I call the public variable set up upon workbook open (you can find that method).  In order to call a private sub you must use.
Application.Run "Global_Variables.SetPublicVariables"
'that is modulename.methodname if you want to pass arguments following
'Application.Run "modulename.methodname", arg1, arg2, etc.
Cheers, Happy coding - WWC