That will depend on what you have volunteered for.
Make sure that your skills are suitable and that you are capable of offering the charity what they require. ASK THEM. Remember, you are not there to study – you are their to contribute your skills.
You might want to consider that your visit is simply a fact finding mission to find a charity to which you can make donations. It is quite possible that it would make more sense to organise charitable donations and funding from your home country.