how to add class to a field in registration infomation?

I'm new to drupal, so apologize for my newbie question.

I have follow this tutorial https://www.youtube.com/watch?v=e9pc63zrzE0 for creating a paid registration form. but i stuck trying to modify the "registration information" form. i would like to wrap some of the input text and also would like to add a class to certain field. is this possible or not? if not, could you give me a guide to accomplish this?

really need help here :)