In the BOL, use the Index tab and enter BACKUP DATABASE. That will not only give information about backing up the database but also about backing up the log.
Basically, you will run
BACKUP LOG dbname TO
You can also search SQL Server Central for more information on backing up and truncating the transaction log. There's alot of information about that already posted.
-SQLBill
BOL = Books OnLine = MS SQL Server Help
Installed as part of the Client Tools
Found at Start>Programs>Microsoft SQL Server>Books OnLine