@oxband
What Mac have you got and what OS are you running? And what version of iTunes, please. (12.3.3 is the latest)
iTunes Music Library is found, as I guess you know, in Preferences > General, and it's the top item. But it may not be there depending on which OS and what version of iTunes you have.
Also, having just 30GB of space on your HDD is way too small. The general recommendation is that you always keep around 20% of your HDD storage space free. It would help us if you mentioned the storage capacity of your HDD.
Ian