The IDE asks you for a workspace when it starts:
The workspace is a folder in which all the files associated with your projects are saved. The IDE offers to create one in <3L>My Designs\workspace, but you are free to change this. The IDE creates structures within the workspace to hold your files in a form that allows the IDE to do a number of clever things. You should not normally need to know anything about this structure, but you should be aware that the physical structure used may not correspond to the logical structure displayed by the IDE. If you do need to discover the actual location of a particular file, you can right-click the displayed file name and ask for its properties.
You can maintain several separate workspaces if you wish and switch from one to another using File/Switch Workspace....