I assume you refer to HTML5 template on storefront.
Code for listing is located in the template file storefront/view/default_html5/template/pages/product/product_listing.tpl
and css in storefront/view/default_html5/stylesheet/style.css
Locate line:
/* listview*/
.list {
display: none
}
and replace with
/* listview*/
.grid {
display: none
}