When cancer is found at an early stage, treatment is often easier and more likely to be successful. So finding cancer sooner rather than later can make a real difference.
Cancer Research UK can help you find out about the key signs and symptoms of cancer. https://www.cancerresearchuk.org/home If you notice any of these, it doesn't mean you definitely have cancer, but it does mean you should get checked out by your doctor. If it is something serious, spotting it at an early stage can make all the difference.
As well as knowing what to look out for, knowing what is normal for you and your body is important. If you do, it'll be easier to notice when something's different.