I need to restore a sql server 2012 into a sql server 2008 express. How can I do?
Short answer: you cant
Restores from different versions of sql server can only move forward
You're going to need to script everything out from SQL Server 2012 (all tables, stored procs, views etc.); then recreate the database structure on sql server 2008
And then you need to get the data from 2012 to 2008; either script out the inserts or use 2012 as a linked server or an SSIS process etc.