One of the ways to truncate the log file is to simply detach the database and re-attach it without the log.

I DO NOT recommend that unless it is a development db that you don't care about.

The proper way to do this is to first backup the TRANSACTION LOG portion of the database by using the following command:

BACKUP LOG <DatabaseName> TO DISK = '<BackupFile>'

And then running a shrinkfile command:

DBCC SHRINKFILE (<FileName>, <TargetSize>) WITH NO_INFOMSGS

If you run the shrinkfile command without first doing a log backup, all it will remove is the empty space in the ldf file, no matter what target size you specify.

Here is the Microsoft support article that explains this.