Problem getting a store

WordPress does not support Stores “out of the box”. You need additional software for that.

If you are interested in free software, check out the following searches through the Theme and Plugin directories of wordpress.org:
https://wordpress.org/themes/search/store/
https://wordpress.org/plugins/search.php?q=store

You can, of course, be more creative with your searches through these directories to help narrow down what you need.

Note that the WordPress Development Team does not support all these plugins and themes; the authors of each one does.

Depends on whether it is a Theme or Plugin. From the WordPress Admin panels, you would choose Appearance (for Themes) or Plugins from the left sidebar, and then Search for and install the Theme or Plugin.

ref. – https://codex.wordpress.org/Administration_Screens