Create a custom plugin in wordpress

http://justintadlock.com/archives/2011/02/02/creating-a-custom-functions-plugin-for-end-users

http://wpcandy.com/teaches/how-to-create-a-functionality-plugin/#.Vt7skJN9634

http://code.tutsplus.com/tutorials/creating-a-custom-wordpress-registration-form-plugin--cms-20968