The technical name for the theory just involving chemicals on a site like Earth is Abiogenesis which you can internet search for a layman explanation.
The Miller–Urey experiment was the first real test of the idea and at the moment there are over 100 experiments running on it just based on paper citations based since 2000AD.
With the discovery of strange lifeforms around black smokers (hydrothermal vents) in the deep ocean the theory of abiogenesis itself had to be widened because those areas have bacteria with lives that revolve around sulfur.
The other possibility is as Bill G said via space and that goes under the name, Panspermia theory which you can again search. That theory still requires Abiogenesis to operate locally but posits that life will be found at many places in the universe.
That should give you enough reading to do to get you started and good luck.
Last edited by Orac; 01/15/16 04:13 AM.
