Evolution needs two things to get going: individuals must differ, and those differences must be inherited. Together, variation and heredity are the raw material on which everything else acts.
Variation
Look at any population and no two members are quite alike. People differ in height, colouring, and countless less visible ways; a field of clover, a colony of bacteria, a population of finches all show the same endless individual variety. Much of that difference is heritable (rooted in the organism's genes rather than merely in circumstance) and heritable variation is what evolution can work with. A trait that varies only because of diet or accident cannot be passed on and cannot drive evolutionary change.
Heredity
Offspring resemble their parents because traits are carried in genes, stretches of DNA passed from one generation to the next. Darwin saw that inheritance mattered but did not know how it worked; that piece came from Gregor Mendel and, later, the discovery of DNA. Genes are the mechanism that lets a useful variation, once it appears, be handed down and spread.
Where new variation comes from
If offspring simply copied their parents, variation would never renew. Two processes keep supplying it. Mutation (random changes in DNA, from copying errors or damage) is the ultimate source of every new variant. Most mutations are neutral or harmful, but a rare few are beneficial, and those are the wellspring of novelty. In organisms that reproduce sexually, recombination then shuffles the parents' genes into fresh combinations each generation, so siblings differ and new mixes are constantly tried.
The setup for selection
Notice what mutation and recombination do not do: they do not aim at anything. Variation arises blindly, with no foresight about what would be useful. That randomness is essential to understanding evolution: the creativity in the process comes not from how variation appears, but from what happens to it next. With a population of differing, heritable individuals in place, the stage is set for natural selection to do the sorting.