This is something you need to think a lot about. Just because you have good intentions it doesn't mean you should come and volunteer, but that is the best thing you can have at the beginning.
I can give you resources so you can find the ideal place for you to apply before volunteering.
Here are some links (I will try to keep this page updated).
http://www.greecevol.info/ - website with a lot of info about where to start, who to contact, etc.
https://www.facebook.com/groups/informationpointforlesvosvolunteers/?fref=ts - Information point for Lesvos volunteers
https://www.facebook.com/groups/204202716585823/?fref=ts - Information point for Athens volunteers
ps: Thank you for wanting to help! I respect you for thinking about this!