Before learning about evolution, we need to go over the basics of genetics, specifically the role of DNA. Deoxyribonucleic acid (DNA) is a molecular blueprint for a living thing. In humans, it dictates everything from your eye color, to your height, to your skin tone.
When organisms reproduce, they get their DNA from their parents. That means a random assortment; you get half of your code from dad and half of your code from mom, and there is little choice in the matter. As you grow, your body reads your DNA like an instruction manual, building proteins, cells, and organs as it goes.
Genetics can be quite complicated — you could spend an entire year studying it in high school or college — but for us, this basic understanding of DNA will be enough.