This FAQ will show you how to set the default Quantity to 1 (instead of 0) on Magento product page:
- Go to Admin > System > Configuration > Inventory > Product Stock Options.
- Click on Add Minimum Qty button.
- Choose Customer Group: ALL GROUPS and enter 1 for Minimum Qty.
It’s as easy as that! Now your product pages will have a default of 1 in the order quantity so your customers won’t have to select this anymore.