Hi,
Since you want online sources, Microsoft has a lot of videos online for you to make awesome apps to windows phone platform. I personally have never used codenameone, but if you want to build cross platform I suggest Phonegap.
You can start by watching the Absolute Beginners series on channel 9
Windows Phone 8 Development for Absolute Beginners | Channel 9
Next you must go through the design principles of windows phone
https://dev.windowsphone.com/en-us/design , it might seem like a good idea to make one app that runs on all platforms but such apps deviate from the general principles and look out of place.
Personally I watched the this series on channel9 , being from web development and android I was able to catch up pretty fast
Building Apps for Windows Phone 8 Jump Start | Channel 9
In case you have any queries you can always post here, there is also msdn
Phone, stackoverflow.
A few more sites I follow windowsphonegeek.com,
.NET Zone | DZone and a few I more I seem to forget.
Hope this helps.