The evidence is massive and far too extensive to even summarize here. It includes DNA evidence, fossils, of course, vestigial organs in existing species, carbon dating, ice cores from antarctica, and physiological evidence.
Any theory in general is a collection of ideas from a scientist who believes that something occurred a certain way for a particular reason. Over the years, many scientists have shared their thoughts with the world on evolution. Also, Gregor Mendel studied the nature of science and how traits can be passed down from generation to generation, potentially dating back from when apes roamed the earth. The Origin of Species can also help you with this question.
We have however found many things to support his theories. Have you seen the evolution of man? From Cro-magnon to Modern day humans? http://goldendome.org/evolutionofman/index.htm There is a lot of random information on this page, but the basis of the evolution of man is pretty solid.
There's alot of debate about this topic and plenty of people on both sides, before becomes yet another flame war can I just say that politely arguing your points and being respectful is the HUMANE and DECENT way of debating, insulting and shouting at people who disagree with you is childish and counterproductive.