It won't hurt learning a bit of backend as well, especially if you're considering learning a frontend framework like Backbone. It will give you a deeper understanding on how your frontend code interacts with the server. I'd start by learning how the request/response cycle works, how to setup your dev environment and how to create simple API's that feed your frontend with json. If you're into python I'd suggest the excellent Flask framework (
http://flask.pocoo.org/). It's extremely lightweight and the documentation is superb.