Most charities would like to know something about you. They might ask you to send your CV or fill out a questionnaire. You could be asked to write a few words about why you wish to volunteer. It should be remembered that the charity is not there to take care of you; they expect you to be contributing without hindering their daily function.
If you are working with children they may ask for suitable references, reasons for which we are sure you will understand.
The organisation you are volunteering may ask where you will stay during your period with them and how you will fund your trip. Charities might wish to know that you have suitable medical insurance cover: the last thing they need is to pay for medical expenses for volunteers.
Charities have to be sure that you are physically fit and able to carry out any duties required of you.