You can create folders within My Presentations to organize your decks. On top, you'll see two tabs to display the presentations: 'Mine' and 'Shared with me'. In the 'Mine' tab, look for this dropdown arrow to access your folders or create a new one.
Click on the '+' button to create a new folder.
Move a presentation to a folder
To move a presentation to a folder, find it and look for more options in its top-right corner. Then look for the option to move it to a folder and select the folder you want to add it to.
Once in the folder, you can follow the same path to remove the presentation from the folder.
Keep in mind that all your presentations will remain visible in the main 'Mine' tab, even after moving them to a folder.