If you already know Java, but you want something that enables rapid development, and uses a more dynamic language, I have just the ticket for ya:

Groovy on Grails[1]

Download and install grails, and you can have your first webapp up and running in a couple of minutes. The Groovy[2] syntax will be pretty familiar to you from Java, but it has dynamic features that Java lacks. Grails also has plugins for tons of popular libraries and extensions, which make adding functionality to your app pretty seamless for many scenarios.

There's a lot of great "getting started" info here:

http://www.grails.org/Tutorials

and the "quick start" guide here:

http://grails.org/Quick+Start

Of course Grails is for the server side... you'll probably still want to bone up on your javascript skills, including possibly something like JQuery. I haven't used backbone.js, but have heard good things about it... might be a cool thing to learn as well; looks like it would be useful for building more advanced client-side javascript stuff.

[1]: http://www.grails.org

[2]: http://groovy.codehaus.org/