Here is an online tutorial.

Installing Python:

• Here is the Python download page. We'll be using version 2.7. (Version 2.6 is okay as well. I switched to 2.7 because this will make it easier for you to install the program.)
• Here is the download page for the Natural Language Toolkit

Learning Python:

• How To Think Like A Computer Scientist is actually an online Python textbook that explains things very well
• A Python tutorial

General Python pages:

• The Python library documentation

The Natural Language Toolkit:

• The Natural Language Toolkit.
• NLTK documentation.