First of all, we highly recommend getting in touch with the project coordinators of your volunteer program. Mostly, they have some good advice, tips, and tricks for you.
Depending on your nationality and the country that you want to visit, you have to arrange your visa. Be sure to research this well in advance as obtaining a visa might take some time. In any case, you will always need an up-to-date passport valid for six months after your departure date.
Tip - Check if you need a transit visa when you have flight connections in different countries.
Once you are set and know about the entry requirements, you can also get some help in obtaining the visas beforehand here.