Look at any population of one species, such as the dogs in a town or the daisies on a lawn, and you will see that the individuals are not identical. There is usually extensive genetic variation within a population. This means individuals carry different alleles of many genes, so they have different genotypes and often different phenotypes.
All of this variation originally arose through mutations. A mutation is a random change in the sequence of bases in DNA. A mutation can produce a new allele of a gene that did not exist before. Over many generations, new alleles spread through the population as organisms reproduce, and sexual reproduction shuffles those alleles into new combinations in each offspring.
Mutations happen all the time, for example when DNA is copied before a cell divides. They happen by chance, and the rate of mutation can be increased by ionising radiation and some chemicals. Because there are so many genes and so many individuals, a population ends up with a large store of different alleles.