Which things I need to know to study relativity theory? And any good book to study it?
Einstein's original introductory work is available free on project Guttenburg. Steve
can you give me the link?
Link at top now
Is it in German ?
One great book that influenced me a lot has been Carl Sagan's Cosmos. You could also search for the works of Michio Kaku, Stephen Hawkings or Neil deGrasse Tyson.
If you just want a basic knowledge of physics, believe it or not a good Science Fiction like Star trek and other Science Fiction books by authors with physics backgrounds. They are more entertaining than a stuffy text book. If you want serious theatrical physics kelseymh has your answer. And if you want something between the two, watch Big Bang Theory, surprisingly they push authors that write physics books for the layman. I am not pushing SiFi because I am a writer surprisingly there are very few physics mistakes that get past our fans. Joe
If all you want is special relativity (comparing viewpoints for observers travelling at different speeds), then you need high school algebra. If you want to learn about general relativity (gravity, acceleration, curved spacetime, cosmology), then you will need to complete college level courses in physics and calculus, including how to solve differential equations.
What level are you starting from, and what level are you aiming at?