Most applications let you press Ctrl+S to save the current document.
If you have previously saved the document (or had opened an existing document), then Ctrl+S uses the same filename as before. No news is good news — if you don’t get an error message, you can assume your save worked fine.
If you haven’t saved the document yet, you will be prompted for a filename automatically. Type in the filename and press Enter.
Ctrl+S is the equivalent to clicking on the File menu and then clicking on the Save command. Note that most programs will show you the Ctrl+S shortcut next to that Save menu command (see image below). Use those visual cues!