Go to Extensions->MyMuse Simple->Categories and choose 'New'.
You must create at least one category for your products. We recommend a top level category with sub categories for Artists and Genres. Some sites create Artists and Labels.
You can assign your products to an Artist and a Genre. These names are arbitrary (they are all Joomla Categories) but it gives some flexability in presentation.
If you are a solo artist, just create a Category for yourself. Your products can have Artist and Category be the same Joomla Category.
You can create menus that point to Categories or to a particular Category, which can list the products under that category in a blog or list format.
When you go to the main MyMuse admin menu, there will be an 'Options' icon at the top right. These are system wide options that set the default Category layout: a list of products as in rows of a table, or a columnar display with several products per row. These settings can be overridden in the menu options.