"Cancer" is a large set of diseases, some of which are caused partly or entirely by inherited genes and some of which are not. Few cancers are caused entirely by inherited characteristics. Instead, some genes make it more likely that a person will develop a particular form of cancer but there is usually some environmental condition that also causes actual cancer to develop.

This can, for example, be that a person has a gene that makes some cells more likely to divide and multiply out of control than usual, but their general immune system cancer-suppression works OK and suppresses it until they have a degraded immune system for another reason. Then they develop cancer.

Or a person may be more prone to cancer, and are then exposed to a further environmental carcinogen, and they develop cancer. Another person with the same predisposition who is not exposed, may not develop cancer.

Even without any predisposition towards being more likely to develop cancers (which is very unlikely - no-one has a perfect genome), exposure to a strong carcinogen may still cause you to develop cancer.

There are also many random factors, such as cosmic rays (literally), that can cause cancer to begin in some part of your body. It's not very likely, but it's definitely possible.

If you don't have any known predisposition towards cancer, then it's probably not something to worry about.

Avoid risk factors like smoking, get your colon checked when you're 50, check your breasts and/or testicles for lumps on a regular basis, and get any odd lumps or persistent unexplained pains, changes in appetite or weight, etc, checked out, but don't go around worrying every day.