I'm new to SFDX, Git and VS Code.  I just step up in windows, VS Code, SF CLI, and Git and tried to clone my repo and pull changes from my sandbox with: 
sfdx force:source:retrieve -x ./manifest/package.xml -u,
but got: 
ERROR running force:source:retrieve:  InvalidPackageDirectory.

I also can't find a file called source_diff which I see in my file at work.  

Any help with a newbie would be greatly appreciated!  TY