Change NonArchive Mode to Archive Mode

Archive Mode is very important.Archive files are the online backup of Redo log files.
Change the mode
Shutdown the database by SHUTDOWN NORMAL OR SHUTDOWN IMMEDIATE;
then
STARTUP MOUNT;
ALTER DATABASE ARCHIVELOG;
Enabling Automatic Archival
by
ALTER SYSTEM SET LOG_ARCHIVE_START=TRUE SCOPE=SPFILE;
then
SHUTDOWN DATABASE;
then
STARTUP

We can check the log mode by issue
SELECT LOG_MODE FROM V$DATABASE;

We can see the log mode and current sequence number by
ARCHIVE LOG LIST;