Is the database in Full Recovery Model? If so, you will need to implement Transaction log backups before you can shrink the log-file size.
If you do not need transaction log backups (in case your full backup is taken often enough that the business is okay with losing the amount of data since the previous backup), you can switch the database to run in Simple recovery model.
You can find out which recovery model a database is in by right-clicking the database, selecting "Properties", and then checking the "Options" tab.

You can change the recovery model at that location, too. However, before you do, I highly recommend you to read up on the different recovery models, and the implications of changing them.
When you are ready to change the recovery model, you might want to read this first.