how to make a joomla plugin?

I am trying to make a joomla plugin, but I have few questions that I haven't found any answer.

What the plugin has to do: add a new field in register form(let's say Cell Number), and on form submit insert that cell number in database.

My documentation is this tutorial.

Questions:

• How do you add a new field in register form? xml file is done, but I am not sure how to write the php code...(please help)

• How do I get the cell number variable from that form? $jinput = JFactory::getApplication()->input; ?

Pleas help me with few tips. Thanks!