Live healthy. Depending on who you are, you may be susceptible to certain cancers. While there is no 100% way to prevent cancer, a healthy lifestyle is a great way to reduce one's risk. It would also be good to find out about family history, and keep in touch with your physician so you can get appropriate screening throughout life.
Reduce risk. You cannot prevent cancer. You can, however, reduce your risk. Maintain a normal weight, diet low in fat and high in fiber, fruits and vegetables, don't smoke, keep protected from the sun. Aspirin has also been shown to reduce cancer risk. If there is a family history of cancer, see a health professional to determine if there is a hereditary risk that requires screening.