This is a very important step to working on a project on AutoCAD as you want your work as you left it the previous time. It is a good habit to develop to save your work in regular intervals, as losing any time repeating the same thing means a lot in any project.
You can save your work by clicking the save button in the quick access bar and then select the destination. This destination is now saved for this particular project and now can be visited again anytime from here and future saves will be done automatically over this file. You can also use Ctrl+S or write save/qsave in command to save.
If you want to save the same file as a different file or want to save it in a different directory, you can use the save as a feature which is right beside the save button on the quick access toolbar.
AutoCAD provides the ability to save your file in a few formats. The default is .dwg, but you can also save it in formats of previous versions, .dws, .dwt, .dxf. This can be done by scrolling through the list – Files of type.