You might have a hard time finding a place to volunteer at the weekends. I personally was able to volunteer with a hospital but I had to fit it around my studies one day a week (for 3-4 hours not the whole day).
Good luck.
(Oh, tiny tip, when you go for an interview you have to show you're confident even if you're not. That's what they're looking for. It wouldn't do you any good saying you're doing it because it would give you confidence. It will. Trust me. But in the interview you have to show you're confident and ready to help out.)
Also smile a lot!
That's the only 'type' of volunteering I'm familiar with but I know there's many more.