Find the pink "Create a New Slide Deck" button at the top of your account. When you click it, it will open a fresh deck with a single join slide and nothing else.
The button will be at the top of your account page at https://slideswith.com/decks/ :
Not seeing the "Create New Deck" button? You'll need to copy a deck first. Once you have at least one deck in your account, you'll be able to see the "Create a New Slide Deck" button
The new deck will look like the below. You'll be able to add slides by clicking the "+" button in the top menu. See our video below of how to build a deck from scratch