When starting from scratch, you'll be presented with the option to add a New Slide. You can also go to "Insert" in the upper menu bar options.
If you're using a template with pre-populated slides, you can add, delete, or duplicate slides by clicking the little arrow on the thumbnails to get options.
On the left sidebar of the Editor screen, you'll see the slides' thumbnails and sequence. You can drag and drop these thumbnails to rearrange the slide order.
When adding a New Slide, Slidebean will give you some preset layouts to choose from and make things easier, or you can also insert a blank slide.
To use a slide from another deck or template, go to add a New Slide, and scroll down on the layout options to find the option to Import slides. You can select one of your decks and then the slide you want to use in the deck you're editing.