After you've done the hard work of setting a filename and determining in which folder the file is stored, saving the workbook is a breeze. Just click the Save button on the Standard toolbar or press Ctrl+S. If the toolbar isn't visible, open the File menu and choose Save.
Remember to save your work often. Keep reminding yourself that the changes made to the file on the screen exist only in temporary memory until you save them. If you exit without saving, you will lose all the changes you made since the last save.