Two ways

First the old, ugly and not to use way (that must be used before epi version 7), set the default value inside admin

Second the new shiny good way: Set default value to true in your code definition for the pagetype:

//Sets the default property values
public override void SetDefaultValues(ContentType contentType)
{ 
   base.SetDefaultValues(contentType);     //Set up your defaults here 
 }

Read more here:

http://world.episerver.com/blogs/Alexander-Haneng/Dates/2012/9/How-to-define-default-values-for-pages-and-blocks-in-EPiServer-CMS-7/

You could also revert the logic, I usually do that.

Ex, instead of having ShowSomething that is checked, you could have HideSomething that is unchecked and check it only when necessary.

BR,

Marija

// This approach works for already created pages as well. If you are using SetDefaultValues, that will work only when you create a new page.