BACKUP LOG MyDatabaseName WITH TRUNCATE_ONLY
DBCC SHRINKFILE(MyDatabaseName_log, FileSize)
(where MyDatabaseName_log is the logical file name, and
The log file has now been shrunk.
Note that the log file will start expanding in normal operation, depending on the way the database is defined. Make sure that the transaction log file is big enough to store all transactions for the normal period between backups. Makes sure that you take peak system loads into consideration when determining the target size.
In normal operation, you would normally never shrink the log file. Regularly backing up the log file will remove the data used in it, and maintain free space for new transaction.