How difficult is it for someone to jump into WP development? Even to develop elementary level apps?
If you already have programming knowledge, you should find it very easy to get started. You'll be using Visual Studio 2010 (for WP7.x) or
Visual Studio 2012 (for WP7 and WP8) and for apps you'll create a project using XAML.
XAML is the design language used by Silverlight and WPF. Although Silverlight is clearly on the way out, XAML itself is still going strong for WP and also Windows 8 Store apps, so I think it's something well worth learning. There are plenty of primers and tutorials on the net which will explain how it works and how to get started (and I have a chapter in my book that covers the basics too) but essentially it's an XML-based description of a user interface. It reminds me of HTML in some ways, but it's much more powerful in terms of its native functionality.
Behind the XAML you can write all the logic for your app in C# or VB. If you've done any kind of WinForms development, you'll feel pretty much at home. If not, the IDE is fantastic and you should pick it up fairly quickly.
Even if you don't use XAML long term, C# is one of the most popular languages in use at present, and anything you learn there is likely to be useful for years to come. (I hope!)
What are the "preferred" tools to develop for WP8? I have heard that Silverlight will no longer be developed, but will still be supported for many years. Does that mean there is a new alternative out for use in WP8 development? How about XNA? There are rumors of that going away too.
The options for WP8 at present are XAML (as detailed above) or DirectX/C++. XNA does appear to have been lost. Which is a *real* shame in my opinion. As far as native platform support goes, the only gaming API now requires C++ programming, which takes it out of the reach of many hobbyist programmers. (Personally I spent many years programming DirectX and C++, but since moving to C# I have no desire whatsoever to go back!)
You can still use XNA for WP7 games, which will work fine on WP8 devices, but if you want to take advantage of new WP8 platform features, that's no longer an option.
There are however other gaming frameworks available that can be used in C#. One such framework is the open-source and freely usable
MonoGame. This provides an API that is virtually 100%-compatible with XNA, but works on Windows 8 and is coming soon to Windows Phone 8 too. This is fantastic both for upgrading existing XNA games, and for writing new XNA games too. I upgraded one of my WP7 XNA games to run on Windows 8 using MonoGame, and the basic upgrade took around 15 minutes (though it needed a lot more tweaking and polishing to run in the Win8 environment after that). I'd definitely recommend taking a look at that for game development.
Then there are other game engines incoming at some stage for both Win8 and WP8, including
Unity and
Ogre, but when these will be available, how good/well supported they will be, and whether they will be accessible from C#/VB, I don't yet know.
Hopefully that'll be enough to get you started anyway, feel free to post more questions if you have them.