I've used Ionic Framework[0], which is basically Cordova + Angular = Mobile Development. If you know Angular, it's a really good place to start, along with ngCordova[1].

If you don't know Angular, then straight Cordova isn't so bad. I actually did some Google Glass development that way. There are Cordova plugins for just about every mobile function you can imagine, with varying degrees of quality. To learn them, I basically took sample projects and examined them, made a few changes, and checks to see what broke.

If you know C#, you can also look at Xamarin[2]. I don't have any experience with it, though.

Edit: formatting

0. http://ionicframework.com/

1. http://ngcordova.com/

2. http://xamarin.com/

I've dabbled but not yet had a project need it. From what I've seen, some places to start:

You mentioned C# so you've probably used at least some Visual Studio, Visual Studio 2015 has Apache Cordova templates and emulators in the default installer now. (You can also download them as an optional separate install for VS 2013.) It's interesting that the VS Android emulator (built for Hyper-V) is the fastest and best Android emulator available right now.

Also, while Cordova tries to be more browser-like than Node-like it may help to have at least passing familiarity with NodeJS. Most importantly you should know NPM (the node package manager) pretty well, as that's the main package manager for Cordova apps as well. (Also, VS helps here too as 2015 has an NPM GUI now.)