Not sure why you need to shrink your log file becuase as SODEEP has already said normally the regular backup of log file (FULL recover module) will truncate the inactive portion of the log file.
However sometimes when a backup is performed the log file is not truncated because each virtual log file that make up the physical log file must be completly inactive for truncation to work or maybe someone as specified the WITH NOTRUNCATE on the backup.
This is why regular backups are required.
However if there is a good reason to shrink the log file by need or design then do the following
USE databasename; GO
-- Truncate the log by changing the database recovery model to SIMPLE.
ALTER DATABASE databasename SET RECOVERY SIMPLE; GO
-- Shrink the truncated log file to 1 MB. -- 1 mb is obviously small - just an example -- therefore insert your own desired size