Relax ask the questions you need to ask they are all very helpful
Don't over think it just be yourself and show them how you are when your relaxed
Just be yourself, open and honest. think why you want to do it. I brought along some references from previous employers too.
Don't, just find a charity that actually cares, don't expect professionalism from everyone like naive old me.
Be yourself. Be willing to put some effort in and help the customers, both with buying goods and arranging for collections. After all, it's all in a good cause.
I would give is to be prepared to the questions that they are asking and also they need to be prepared to do anything that they are told.