Darwin proposed natural selection as a mechanism for evolution.
The process humans use to change a species by selectively breeding for specific traits is called artificial selection. Darwin noticed a lot of variation in domestic plants and animals.
Watch the Crash Course video, Natural Selection.
After watching the video, read these three pages on the Berkley, Evolution 101 site:
The four main principles of the theory of natural selection are:
It is also important to remember that natural selection can act only on traits that already exist.