The third parameter to form_dropdown(), which you are declaring as 'null', is where you set the default.
I generally code this as a variable, which either gets populated from a database field, or from the user selected value for the field. In the latter case, you have named the select 'bedrooms', so you would set the default as:
$var = set_value('bedrooms');
echo form_dropdown( "bedrooms", array( null, 0,1,2,3,4,5,6,7,8,9,10), $var, $form_prop );
When setting it from a database field:
$var = $db_rec['bedrooms'];
echo form_dropdown( "bedrooms", array( null, 0,1,2,3,4,5,6,7,8,9,10), $var, $form_prop );