Your customer account and store admin account are essentially the same. We call those user account. Store admins are user accounts with "Administrator" role assigned. So you could add "Administrator" role to anyone to make he a store admin. Thus to create a store admin account, you just go to your store home page and register an account just like your store customer. Then you log off and log back in using any other existing store admin account. When we first set up your store we create a default store admin called "admin". Use Search Customer to find the user account that you just created. In user detail page find "Roles" on the bottom. There is a "Add Roles" button. Click that button and check the "Administrator" checkbox. Click "Update" to save the change. Now the user account becomes your store admin.