If you want to get started with Windows Phone 8.1, there are 2 big options:
“I’m completely new to Windows Phone development.”
You need the Absolute Beginner Series for Windows Phone 8.1 It is a complete course in getting started for Windows Phone 8.1. It is 30 videos that are 10-20 minutes long.
You can get the source code for all the projects here.
“I’ve done some (or lots) of Windows Phone before & I want to know what is new in this release.”
You need the Building Apps For Windows Phone 8.1 Jump Start. It is 23 videos from 30-60 minutes long and a LOT of demos.
Demo source code is available here.
“I want to get an overview of the features available in Windows Phone 8.1”
Watch the first session of the Jump Start. Watch other sessions based on topics as needed.
“I’ve built for Windows 8.1 and I want to build Windows Phone too”
Congratulations, you already can. Windows Phone 8.1 allows Universal application develompent. Basically, everything you’ve already built can move over to Windows Phone. Your XAML will probably be a challenge to make a 1-to-1 correspondence, so you might want to re-work some of your screens to fit the phone form factor. The difficulty level is comparable to building apps across iPhone and iPad.