To create your first Numbers spreadsheet within iCloud go to the main Numbers file management screen at iCloud Numbers
Now decide where you would like to store the new spreadsheet. It can either be stored in the main iCloud Numbers screen, or preferably in a suitable folder such as Documents, or a sub folder within that. A new folder can be created using the New Folder icon in the top row, or by right clicking and selecting New Folder.
The new folder name will be highlighted and can be changed to something suitable, such as Documents.
If the name isn't highlighted, or you need to go back and change it, then hover over the folder with the mouse cursor and a small circle with three dots will appear. Click this to open a sub menu with the Rename Folder option.
Click and enter the new folder now so that the spreadsheet will be saved there by default. Note that spreadsheets can easily be moved into folders once they have been created and saved.