[gcheong]

I would start with the tutorial http://code.google.com/appengine/docs/python/gettingstarted/. They also have a Java API if that is more to your liking. See if you can follow and understand the tutorial and fill in any gaps where you seem to be lacking in knowledge or understanding with other sources. If you know any programming language I think the Python is relatively easy to follow.

[quizbiz]

If I don't know any Java or Python, which would you recommend? Which is more similar to RoR?

[oomkiller]

Since AppEngine runs Java, that means it can run JRuby, which also means it can run Rails. Use it if you like. There are a couple of special considerations when using Rails on App Engine, although I am not familiar with them. Finally, you should remember that you're stuck with their datastore, so you can't use ActiveRecord (at least as far as I know, there may be a gem that lets you)

[nir]

Python should be easier to pick up. I knew very little Python before using GAE, looking back there was hardly a learning curve. It's almost like writing pseudo-code. (Though somehow not as fun to code as Ruby, for me..)

[gcheong]

I don't know RoR but Ruby is similar to Python from what I've seen.