The main thing that affects how likely you are to get cancer is age – older people are more likely to get cancer than younger people. The other main thing is smoking – smokers are much more likely to get cancer than non-smokers. Being obese can also make people more likely to get some cancers.
Other than that, there are lots of different things that can affect how likely someone is to get cancer and each of the cancers has a different set of things that make them more or less likely. For example, too much time in the sun without sunscreen can make you more likely to get skin cancer, but won’t make you more likely to get bowel cancer.
Some of a persons risk of cancer is inherited from their parents. There are some genes that make people much more likely to get a certain cancer – for example, having the BRCA1 or BRCA2 genes make people much more likely to develop breast cancer. Fortunately, ‘high-risk’ genes like these are quite rare so only about 1% of cancers are caused by them.