It's a usual mistake that fresh volunteers are given too difficult tasks at first. Helping for example handicapped, injured or homeless people can be very stressful for someone who is not prepared enough for the job. It means that their desire to help can easily transform into finding the way to escape. So you have to think it over very carefully what kind of duty you should give them. It is important to find a case where your people can easily get positive feedback: trash picking, park cleaning, interesting cultural programs. So let's start with something that is not too difficult to deal with. After they realized the beauty of volunteering they will be more open to other possibilities.