This tutorial covers some CodeIgniter basics, if you would like to start with more of the basics stuff I would suggest looking at the links below the article.

The CodeIgniter TellAFriend tutorial covers usage of two very common classes used in the form of libraries and helpers - Form Helper and the Validation Library. The TellAFriend tutorial also covers some basics of the email library of CodeIgniter. For your convenience you can download the tutorial files from github. Do remember to read the readme.txt file with the archive.

CodeIgniter TellAFriend Tutorial: Form, Email and Form Validation using CodeIgniter.