The most common presentation of testicular cancer would be a lump on your testicle that you would feel during a self-exam. Given your medical history, it would be a good idea for you to do self exams regularly, and to talk to your family doctor about your medical history, and have him do a testicular exam at your annual physical. Your doctor can also give you and your parents some information about testicular cancer that might help you understand it and know what to look for.
You can find information on how to do a self exam here: http://www.tc-cancer.com/about/self-exam.html
Being knowledgeable about what to look for is the best defense, but in the meantime, calm down, and enjoy being fourteen! No reason to make yourself crazy with worry about something that, statistically, will probably never, ever happen.