Answer 1

Depends what kind of application you're trying to build.
For desktop applications, you can use Tkinter or PyQT which work on Linux, Mac and Windows. GTK which is most popular on Linux. You can also develop you application using pygame, which is also platform independent.

Creating a CLI (command line interface) is the easiest kind of application you can make with python since python scripts can be executed via the command line.

Or perhaps you wish to build a web application in which, by far, the most popular web application framework is Django. Other web application frameworks for python include: Flask, and pyramid.