To save a Excel Spreadsheet, you can press Control + S, give the spreadsheet a name and press Enter.
Another way to save is to use F12, which is Save As. You put in a name and then you can Tab around the Save As dialogue and choose where to save the spreadsheet. You can then Tab back to the name and press Enter to save. It will save in your default place, if you do not choose a save location.
Note: In Office 365, it will automatically save to your OneDrive, but you can change the default save location in the Excel Options, under the Save section. There is also an option in here to turn off the Backstage when opening or saving. This simplifies the saving so that when you press Control + S on a new spreadsheet, you can just press N if you do not want to save it.
Once your spreadsheet is named and saved for the first time, from then on you can just press Control + S to save the changes.