Volunteer information is extremely helpful for those who wish to help others. There are certain risks and rewards that come with being a volunteer so check out these articles from HowStuffWorks to make sure you have all the necessary volunteer information
While working in the United States can be difficult for a foreigner, volunteering is usually pretty simple. Forget the hassle of work visas. As long as you aren't being compensated for your volunteer work, you shouldn't need one.