How can I create relationships in multiple tables on rails?

I'm new to rails and learning relationship between tables. This is what I want to create. Users wont be able to post into classes or videos just add them to their table.

example: I'm a student and I sign up for a user account. Then I choose the classes I want HTML and CSS. And then I'm only able to watch the videos inside those classes.

Users->classes->videos

(USERS)
username
password
classes - signs up for multiple classes

(classes)
HTML
CSS
Javascript
PHP

(inside of each class theres videos)
video1
video2
video3