Possible duplicate of
Deploy repackaged android (BB10) APP in Blackberry Enterprise Server within workspace
You can't reuse any of your code.
The best option depends on the type of application. If it is reasonably complex, with interaction with on device stuff like location, then Cascades/C++. Alternatively, you might consider html5. Have a look round here for more:
https://developer.blackberry.com/
Playbook is another curve ball. If you have to support that then you can't use Cascades.
And to be honest, there is a certain amount of personal preference involved here too - what do you like writing in?
Just a FYI, it took me about 6 months, part time, to feel fully comfortable with C++/Cascades. This was from nothing, no C, no C++, no experiences of GUI design tools and a different IDE. But I created my first app, a really simple one, in two weeks.