Hola
There are different ways of doing this.
You can send stuff ahead to the Post Office in Santiago. The service is called Lista de Correos. Post your box addressed to yourself to one of the post offices. Mark it "Lista de Correos". The nearest Post Office to the Cathedral is:
Rua Orfas, 17 15703 Santiago de Compostela
However please note they will only keep your box for 15 days maximum.
Otherwise you can send it here:
The Pilgrim helper will store your luggage in Santiago for up to 60 days. The name of the facility is Pension Badalada.
http://www.badalada.es.
Charges are:
Small box (317 x 215 x 125 mm) €15 for 60 days
Medium box (390 x 290 x 190mm) €20 for 60 days
Large box (590 x 390 x 390mm) €25 for 60 days
Address your luggage as follows:
Pension Badalada
ATT: [Your name here]
Rua Xelmírez, 3
15704 Santiago de Compostela
OR - forward your box to the hotel you will be staying at in Santiago and ask them to store it for you.
Good luck!
John