Introduction to CodeIgniter and ExpressionEngine
Jeffrey Way's tutorials on nettuts are really good:http://net.tutsplus.com/videos/screencasts/codeigniter-from-scratch-day-1/
You might find this this tutorial helpful for migrating a Codeigniter app to ExpressionEngine:
http://jadb.wordpress.com/2011/11/15/migrate-codeigniter-app-to-expressionengine/
(this assumes you know a little something about both CI and EE)