It sounds like you don't backup your transaction log. Have you ever done a full backup? If not, DO IT NOW!!!

If you have done a full backup, backup the transaction log. This shrinks the log file (not immediately though). If you want to shrink it right away, you can BACKUP LOG dbname WITH TRUNCATE_ONLY. (That can be run even if you just backed up the log). You should immediatly do a full backup after truncating the log. Or you can shrink it using the DBCC SHRINKFILE command.

-SQLBill