~4.6 billion years ago certain gases (all of which were present on earth) are struck by ultraviolet light (from the sun) they produce amino acids (the basis of things such as dna, rna, muscles etc). These formed the first "life". This early "life" was a simple collection of dna that absorbed the sunlight and the early gases and made energy. Eventually these formed the first cells and bacteria and began producing oxygen (16x more powerful than early H2S photosynthesis). With this abundance of life a new class emerged, this class was able to eat other life. We call these "sponges". These sponges evolved muscles and a nerve net becoming polyps and jellyfish. These evolved stomachs, hearts, lungs, legs, hands, feet, etc. and created the vast variety of life that populates this great planet of ours.
Alternativley: 6000 years ago a large, immortal deity created life in 7 days, buried all the dinosaur bones and layered out all the rocks so as to make the earth seem really old in order to "test our faith". eventually one of his angels got pissed off, became a snake and told adam and eve to eat an apple from a tree that the deity had placed there the deity got mad and kicked everybody out. Thus humanity came about.
I have layed out the most popular thoeries, the evolution one and the mainstream religions' (Judaism, Christianity and Islam) one. Pick for yourself.