Create a directory somewhere, and call it PythonWebServer. Then download the appropriate Python-based web server from the link below and unzip it into the new PythonWebServer directory.  

The Python Web Server (and associated files)

When unzipping, be sure to allow the creation of new directories.  When done, you should have 2 new directories beneath PythonWebServer called html-stuff and cgi-bin with some files already in each of them.  If that's not the case, redo the unzipping to fix that.

For Unix/Mac systems: After unzipping, change the file permissions of all Python files that you've unzipped to allow execute permission.  Also the webserver.bat  file. You'll find Python files and webserver.bat in the PythonWebServer and PythonWebServer/cgi-bin directories.

In the future:
-- HTML files: put your .html files (those that do not involve any Python programs) into the html-stuff directory
-- Python programs: put the Python programs that the web server should execute into the cgi-bin directory. 
    a) However, if there are data files that your Python program opens, put them into the main directory (the one that the PythonWebServer.py file is in), but don't use add any "path" to the filename when using the "open(filename,'r')" command even though the data file is in a different directory than the program file.
    OR...
   b) You can put data files into the same directory (cgi-bin) as your Python programs, provided that your program changes its directory (using os.chdir()) to the cgi-bin directory.  The programs I give you will automatically do that.

On Windows: Find the PythonWebServer directory (using a file manager), and double-click on the file: webserver.bat.  A black window should come up, with the words: "Serving HTTP on 0.0.0.0 port 9000"  If that doesn't happen, then for some reason, Windows can't execute your python programs automatically.  Try to fix that, if you can.
OR...
Get into a CMD window, navigate to the PythonWebServer directory and execute the file: webserver.bat.

On Unix/Mac: Open a terminal window and navigate to the PythonWebServer directory and execute webserver.bat.  If you're running the bash shell, then bash webserver.bat.

In your browser (e.g. Firefox, Chrome, Safari, etc), go to the following web page: "http://localhost:9000".  You should see a web page that will step you through some tests.  If you don't, then there's trouble in webserverland.

In the black window where you started the web server, there should be a record of all of the pages it served.  Press Ctrl-C (^C).  Possibly more than once. That should shut down the server.