Oracle version : 10.1.0.2.0 (64b)
Platform : Sun Sparc 5.8 (64b)
Server : Solaris Fire v20
Storage : SAN
before enabling archivelog mode take complete backup after database shutdown (cold backup).
1. backup spfile : create pfile='/path/pfile.ora' from spfile;
2. enable archive destination
sql> alter system set log_archive_dest_1='Locatin=\path' scope=spfile;
2. enable archive destination
sql> alter system set log_archive_dest_1='Locatin=\path' scope=spfile;
   sql> shutdown immediate;
3. perform complete backup
5. connect exclusive mode
$sqlplus "/as sysdba"
          sql> startup mount;
sql> alter database archivelog;
sql> alter database open;
sql> archive log list;
          Database log mode Archive Mode 
          Automatic archival EnabledArchive destination \path\archive
Oldest online log sequence 5
Next log sequence to archive 7
          Current log sequence 7
6. after enable archive, better to another complete backup
 sql> create spfile from pfile='/path/pfile.ora'
sql> shutdown immediate;
$ cold backup
 sql> startup
Note :
  log_archive_format must be in this format '%t_%s_%r
  %t - thread number
  %s - sequence number
  %r - resetlog id
Error : 
 SQL> startup pfile=D:\oracle\admin\ORCL\pfile\initORCL.ora
 ORA-32006: LOG_ARCHIVE_START initialization parameter has been deprecated SQL> startup pfile=D:\oracle\admin\ORCL\pfile\initORCL.ora
ORA-32006: LOG_ARCHIVE_START initialization parameter has been deprecated
ORA-19905: log_archive_format must contain %s, %t and %r
ORA-32006: LOG_ARCHIVE_START initialization parameter has been deprecated
ORA-19905: log_archive_format must contain %s, %t and %r
Metalink Doc:
      Doc ID: 274302.1
      Doc ID: 371139.1
      Doc ID: 69739.1
      Doc ID: 420371.1
Good Luck!