Somewhat more approachable than the official website is
CodeRanch's Beginners Resources. The links there cover basic Java and things you'll want to understand (classpaths, singletons, etc.) The motto there is to be friendly to people new to Java and they've kept things accessible.
Another
free resource is the book "Thinking in Java."
Disclaimer: I moderate their in addition to volunteering with FIRST. My hobbies keep me busy.