When you create the field, you can set a DEFAULT value:
SQL DEFAULT Constraint[
^]
But, for a boolean field what value it should have is use specific: for a BIT field 0 is false and 1 is true - but your application will determine which way existing records should be set. If it's a "deleted" indicator, then if all existing / new rows should be considered "undeleted" then you'll want false for example.
We dont; know how you plan to use this, so we can't tell you "do this"!