In Magento you can change the default sorting attribute for product listing page from Magento admin.

To set the sorting attribute login to Magento admin and follow the following steps:

1. Go to Admin >> System >> Configuration >> Catalog
2. Expand the “Frontend”  Section
3. Make selection from the “Product Listing Sort by” dropdown
4. Click the “Save Config” button.

If you have fresh Magento installation then you will see the following options available for in the “Product Listing Sort by” dropdown –  “Best Value”, “Name”  and  “Price”.

If you want the default sorting to be by some other attribute not in this dropdown then you first need to make it sortable. To do this follow the following steps:

1. Go to Admin >> Catalog >> Attributes >> Manage Attributes
2. Click on the attribute row to edit it
3. In the attribute properties page, scroll down to find the heading “Used for Sorting in Product Listing” under “Frontend Properties” section
4. Select “Yes” in the dropdown
5. Click the “Save Attribute” button.

Now this attribute will appear in the “Sort By” dropdown on frontend as well as in the admin “Product Listing Sort By” dropdown.