There are two general approaches to a volunteer sign-up page.
The first approach is to publish a generic simple volunteer page. This page would simply ask people to volunteer, then rely on staff to contact the person to involve them in the volunteer organization. This approach tends to maximize online signups. It then creates the opportunity (or burden if you prefer) for staff to manually contact the volunteer, earn a deeper level of commitment and update the new volunteer's record with the correct data.
The second approach is to create allow the visitor to define, up front, what volunteer activities they are and are not interested in participating in. Obviously, the benefit of this is that more data is handles automatically, without the need for intervention or personal contact. The alternate argument is that it des not give you that opportunity and it allows your visitor to preclude themselves from needed tasks.
It is our experience and opinion that a better practice is to use the first approach. This does require you set up a good human process that includes immediately responding to new volunteers with a skilled person that can maximize this valuable resource.