First question:

which python though its usually /usr/bin/python for the 2.7

Second question:

From a terminal & python2.7: python2.7 yourfile.py.
Simailarly for 3.2: python3.2 yourfile.py though 3.2 isn't installed by default. (You can apt-get install python3.2.)

What python yourfile.py will do depends on which alternative is used for your python interpreter. You can change that by issuing update-alternatives python as root (or by using su).

Third question:

Environment variables are shell dependent, though you can write them out with echo $variable and set them with variable=value (from bash). The search path is simply called PATH and you can get yours by typing echo $PATH.

I hope this was helpful.