How did Anakin become Darth Vader?

I watched Star Wars: Phantom Menace and Qui-gon told Obi wan to train Anakin but why did Anakin turn into Darth Vader? Also, some people say," they hate the first episode of Star Wars. Why? I thought it was pretty good. And, Jar Jar Binks in the 1st episode is pretty much useless. Why did George Lucas add him?