Although we don't know how to prevent someone developing cancer, we do know that you can reduce your risk by making lifestyle choices. For example, not smoking, avoiding sun damage to your skin and not drinking heavily can all reduce the risk of developing some cancers. Also, eating a well balanced diet, that includes five portions of fruit and vegetables, and taking regular exercise may reduce your risk.
There are a number of questions and answers about how to reduce your risk of developing cancer that you may find helpful to read.