How does someone get leukemia i'm very confused about it?

Bad luck: There is a small genetic link but mostly it is bad luck. Some minor risks include previous exposure to chemo or radiation or chemicals.

Random: We do not know definite causes for leukemia although exposure to higher doses of radiation (such as accidental exposures like chernobyl) are associated with an increased risk. One theory of leukemias and other cancers is that random genetic mutations or mistakes lead to cancer.