To create a folder, find the "Add folder" button on the left sidebar of your account homepage.
Name the folder and add a short description if you like. In order to edit or delete the folder, just click on it and look for the Settings button, next to its name. Deleting a folder won't affect the documents contained in it, as they will still be accessible in your main dashboard.
In order to move a deck into a folder, go find the deck among your presentations and hover over it. There, click the Ellipsis button on the top-right corner of the deck, and go to Folders. Just select the folder you want to move the presentation into and hit Save.
Remember, the presentation will also remain available in your main dashboard screen.
To remove a presentation from a folder, access the folder and hover over the presentation. Again, click the Ellipsis, go to Folders and just deselect the folder in which the presentation is currently stored.