What types of volunteer work should I do to prepare for the career I want to pursue?

I am not in the medical field however when I was in high school I planned on going to medical school. To prepare I volunteered at the children's hospital nearby. It was a very valuable experience. I was able to work with hospital administrators, medical professionals, and patients themselves. I felt like I was really making a difference and I was able to see the medical profession first hand. I recommend checking out local hospitals in your area to see if they have volunteer opportunities.

As others have shared volunteering with an organization that works in a field your wish to study is a great way to learn if that is what you really want but continue volunteering.   Most large companies and organizations have an employee volunteer program.  Take advantage of their program and volunteer with team members to build your network, volunteer to build certain stills, volunteer to do good.  Whatever motivates you I know you will learn something.

Adding on the great answers above, boil down what you think are the desired skills to work in the medical field; they could be directly linked to medicine or more general. For example, leadership is a vital skill in any profession, if you volunteer with a group unrelated to the medical field but are able to sharpen your leadership skill, you're separating yourself from the competition.

In addition to volunteering with in a hospital, I would recommend volunteering with the elderly, children, or with handicap people.

Volunteer anywhere you can. Doing so will help build an altruistic mentality. In addition, volunteermatch.org has a lot of medical related volunteer options.