Where can I find a good tutorial on programming with Lejos?
The makers of LeJOS maintain a blog on WordPress: https://lejosnews.wordpress.com/ where they also give tips and write howto's. I think that is the best place to get started, apart perhaps from the LeJOS.org wiki pages which are located here: https://sourceforge.net/p/lejos/wiki/Home/. 

Those wiki pages tend to have some inconsistencies between versions (some of it is not yet updated to the latest version of LeJOS EV3) but when I got started with LeJOS 2 years ago I found it quite easy to get going using those pages. 

Of course, all of these pages assume you understand how to write programs in Java - otherwise you'd need to look for a Java tutorial, like the ones published by Oracle at https://docs.oracle.com/javase/tutorial/.