First of all, depends on the way you selected the options.
As stated previously, if you selected windows authentication then "sa" is disabled. If you have mixed you still might have to enable the "sa" and "sa" can not be blank. There is no default password for "sa", if SQL express comes with a applications ask the applications support team.
You can put the SQL Server in single user mode and then any member of computer's local administrator can gain access to SQL server as sysadmin.
Go to SQL Server Properties/Security tab and change the mode to SQL Server authentication mode
Go to security/logins, open SA login properties
Uncheck "Enforce password policy" and "Enforce password expiration" check box there if you decide to use weak password
Assign password to SA user
If you get "Cannot set a credential for principal 'sa' . (Microsoft SQL Server, Error: 15535)" then "Map To Credential" check box on the General tab of the Login Properties - sa dialog box.
If “User is not associated with a trusted sql server connection"
In SQL Server Management Studio, Right-click the Server name, select Properties > Security
Under Server Authentication, select SQL Server and Windows Authentication Mode
The server must be stopped and re-started before this will take effect.
Make sure the sa (at status tab of sa properties) is enabled after restart the server.