George Lucas grew up watching TV serials, like Flash Gordon, which would often give a brief recap of what was going on, or some intro into his current situation, then jump straight into the action.
There was no time spent getting viewers familiar with the characters and the surroundings. It was assumed that the viewer knew about spacecraft and laser guns and aliens.
Lucas wanted to create a Flash Gordon film, but couldn't get the rights to it, so he decided to make his own "space opera" with a similar vibe to it, which is why you have the opening text crawl, and it jumps straight into the action. It doesn't bother explaining what astrodroids are, or protocol droids. You get enough info through context, or it just doesn't matter to the plot.
Originally, he wanted to do a trilogy, because as he wrote the story, it became too much for one film. He started at episode 4, to give the feeling of being immersed in the middle of the action. However, there was no guarantee that the first film would do well, so he had to condense his ideas for the three films into something that worked on its own.
I believe that Endor was meant to be inhabited by wookiees. Because Lucas wasn't sure if he'd ever get to show Endor and the wookiees, he included Chewbacca. Later, when he got to have Endor, he didn't want it to be Chewbacca's planet, so he made them smaller and more bear-like and called them Ewoks.
Rather than starting the first film from episode 4, and potentially confusing the audience, it was simply called "Star Wars". No reference was made to episodes or subtitles.
It wasn't until after the release and success of the next film that the "Episode IV: A New Hope" subtitle was added.
There is an interview where he said the prequels had been planned since 1981.
This is from the same interview, though;
Lucas: Also another misconception is that Yoda teaches Jedi, but he is like a guru; he doesn’t go out and fight anybody.
Kasdan: A Jedi Master is a Jedi isn’t he?
Lucas: Well, he is a teacher, not a real Jedi. Understand that?
Kasdan: I understand what you’re saying, but I can’t believe it; I am in shock.
Lucas: It’s true, absolutely true, not that it makes any difference to the story.
Kasdan: You mean he wouldn’t be any good in a fight?
Lucas: Not with Darth Vader he wouldn’t.
And I'm sure I've seen interviews where he's said that he didn't intend on doing prequels because it contradicts the idea of starting from the middle, and also that the original theatrical versions had been lost or destroyed, then they were released as a special feature on DVD. So take what he says with a pinch of salt.