On the listing page of each category, the products are sorted by default by Relevance, i.e. by the best-selling ones. Alternatively, you can select sorting by "Product name" at the top of the results list: the products are displayed in alphabetical order based on their title. Since the title of each product always starts with the SKU code, this sorting effectively corresponds to sorting by code.
These are the only two sorting criteria available: at the moment, it is not possible to sort by price, availability or other parameters.
If you want to narrow down the results instead of changing their order, you can use the filters available in the left column of the listing page. To find out more, read Can I filter products by brand or by technical characteristics? To customise the display of the results, read Is it possible to customise the display of the search results?