Creating Windows Metro style apps with Java?

You can only create metro style apps with C#, VB, JavaScript and C++. There is currently no support for authoring metro style applications with Java.

You cannot make metro style apps with Java.

For the other part of your question, regarding starter's guide and which language is easy, MSDN has a starter's guide which helps building new metro style apps, this, this and this should be helpful.

And which language is easiest, well it depends on the developer, someone who has been developing .NET or WPF or Windows Forms applications maybe more comfortable with he familiar environment and may prefer developing using C++ and XAML or VisualBasic, depending on what he has been using before, while someone coming from web development may prefer javascript.

For new developers, i would recommend using C# and XAML, but again it is my choice, and not everyone will agree with it.

Here is another link which points to guides at MSDN, and adds some additional detail.

I believe you will not be able to use Java and WinRT. The best option for you to try is C#, because this is the language that is closest to java, in my opinion.