Python is a good choice as a starting language, and there is Quickly, which helps you to create graphical applications easily. Quickly helps you to set up everything you need to develop a GUI application.
Check out how to get started with Quickly on the Ubuntu App Developer site.
To install Quickly type:
sudo apt-get install quickly
You can then get a tutorial by executing:
quickly tutorial ubuntu-application
To create an application just type:
quickly create ubuntu-application Testapp
which will create an application called "Testapp" using the "ubuntu-application" template.
You can design the graphical user-interface by executing
cd testapp
quickly design
which will open Glade, the GUI designer.
To change the actual source code just type
quickly edit
Learning how the Python language works is essential to having a productive and easy experience developing in the Python, GTK, Quickly, Ubuntu environment. Here are some resources:
(an easy introduction to the Python programming language, free)
(an advanced book, free)
(an advanced book, free)
Also, bookmark the PyGTK 2.0 Reference Manual.