How do I add an input field to the registration page?

Hello,

I apologize if this is redundant; I found this question asked a couple of times, but I was unable to find a suitable answer that made sense to me.

Basically I just need to add a field to the registration form and account information that asks for a phone number (optional) with a checkbox after it asking whether or not the user would like to receive SMS text updates.

What would be the simplest way to achieve this?

Thank you!