If you are a beginner and somewhat comfortable with C# then switching to Java is a good progression. The Java language is very popular across most fields and is probably the language that stands head-to-head with C++, but for different reasons such as speed verses ease of use. Also, there is the Android platform everyone is talking about, so games-wise its no longer a poor choice.
As for learning Java, then just go with something like SAM's "Teach Yourself Java in 21 Days" or if you really want to take the bull by the horns, consider an evening, college or university module. If you are in the UK or nearby then I would recommend the Open University's M250 module which takes the beginner programmer from a gentle start in Java to a surprising depth of knowledge one could learn in a mere 9 months. LOL, I was a bit of a die-hard C++ programmer when I took that module(actually it was M255 its previous presentation) and even I was won over by the module! But that's just another option to choose from and might not be for you. Education is expensive these days and all....