Be sure to keep a copy of the form so that you can refer back to it at any time. Yes, you can withdraw your consent at any time, but it's best to take the time you need before you start treatment to make the best decision possible. Questions may come up during the study that you didn't think about before you agreed to participate. Don't hesitate to ask these questions of any member of the research team to get answers and resolve concerns. That depends on the study.
Most studies are available at community hospitals as well as at the central university hospitals.
Ask your doctor. They can give you clinical trial information over the phone, or send it by mail. Call a cancer center in your area. The National Cancer Institute has a site specifically devoted to clinical trials. All of the major cancer centers also have web sites.