You're opening an existing workbook created in a previous version of Excel. Click the Microsoft Office Button in the upper-left corner of the window. There you'll get the same commands you've used in the past to open and save your workbooks.
Before getting down to business, notice that this menu is packed with useful items. For example, here is where you'll find the program settings that control things like turning the R1C1 reference style on or off, or showing the Formula Bar in the program window. Click Excel Options at the bottom of the menu to access the options.
In previous versions of Excel, you could set such options in the Options dialog box, opened from the Tools menu. Now many of those options are here, where they are more visible, and conveniently close at hand when you start work on old files or new ones.
Now, getting back to that workbook, click Open, select the workbook you want, and then click Open.
That's all you have to do to open a file created in a previous version. You're ready to get to work.