Answer 1

I think it depends on what you will achieve ;)
A simple calculator doesnt need a lot of knowledge, a fancy handy game needs a lot of knowledge and experience.

But a basic knowledge of Java (if you use Java) isnt a bad idea. Grab some youtube videos and start with simple apps like a calculator or a calendar.

Answer 2

I will recomnd you phonegap, which allows you to creat apps for android and ios if you already know javascrpit, html and css