Going off of what I can recall off the top my head...aurebesh has its origins in ROTJ. You can see some of the characters on the DSII's displays. From there the roleplaying game company West End Games adapted the characters taken from that screen and turned it into an actual alphabet and established that as the galactic standard. I recall reading an interview with one of the designers who worked on ROTJ who actually designed the original, and he said that the production actually had their own version written up but it's never been released. Since the release of that roleplaying material, the rest of the expanded universe, the prequels and even the special editions have used it as the official Star Wars alphabet.
As for the languages in Star Wars, that was all the doing of Ben Burtt as zombie has said. He liked to study obscure exotic sounding languages and would scavenge words from them to use. Apparently Ben fell in love with the process because a few years ago he actually wrote an entire travel and phrase book for the Star Wars galaxy. If you'd like to get into all the details of his work I'd recommend you pick up that book. It has a great in depth article he wrote chronicling all the work he did for the films.