If Its a static list of items, you can set the default value on the component itself by entering a value into the Default Value field
e.g. to set "Item 2" as the default:
or using the Rules Editor:
If its a dynamic list, you can set the value property [2] once the list is initialized [3]
[2] Adaptive Forms Class: GuideDropDownList
[3] Adobe Experience Manager Help | Adaptive Form Expressions