After installing Python on your system successfully, you can start the interactive Python prompt by typing python in the command line and press <enter>. It will show you some context information about Python similar to this:

Python 2.7.2 (default, Feb  1 2012, 00:28:57)
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

Those three >>> in the last line indicate that you are now in the interactive shell of Python. Type for example:

print("Hello world")

Press <enter> and see what happens. You will now see the phrase "Hello world" appear and then Python will bring you back to the interactive input, where you could enter another command now:

>>> print("Hello world")
Hello world
>>>

To leave the interactive shell, press ctrl + D.

But you don't want to type everything into the Python shell every time. Instead having a file with commands and handing that to Python to execute it would be much better. In order to do that you can just pass a file name to the python command in your shell and it will execute that file. Let's try that. Create a file hello.py in your current directory with your favorite text editor and paste the print command from above. Now save that file, go back the command line and type

python hello.py

When pressing <enter> now, the file is executed and you see the print as before. But this time, after Python executed all commands from that file, it exited instead of going back to the interactive shell.

And now we are all set and can get started with turtle.