Set the LOG_ARCHIVE_START parameter to TRUE in the initialization parameter file.
Automatic archive is the default when ARCHIVELOG mode is set. No further actions are required.
Set the LOG_ARCHIVE_MAX_PROCESSES parameter to n (where n is the number of Archiver process) in the initialization parameter file.
Immediately after setting the database in ARCHIVELOG mode, in SQL*Plus select automatic archiving using the syntax : ALTER DATABASE SET LOG_ARCHIVE_START=TRUE