One really helpful resource is the
Javadocs. Navigating it is pretty simple, you click on the class on the lower left and it displays information about the class. You'll probably want to look at the Victor and RobotDrive classes for what you're specifically using right now.
If you need specifically "How do I Java" rather than "How do I write robot code" I recommend
Oracle's tutorials. It takes quite a bit of effort to learn if it's your first language, but it'll be significantly easier if you have prior experience with text-based programming (eg. not scratch).