Try these links:

https://khanacademy.zendesk.com/hc/en-us/categories/200175830-For-Parents

https://khanacademy.zendesk.com/hc/en-us/categories/200175820-For-Teachers

Second question first: if you have students who have added you as a Coach, and if you create a class on Khan Academy and add those students to the class, then you are already the teacher of that course.  "Coach" and "teacher" are interchangeable terms (I think they picked "Coach" because they might also be tutors or administrators or other titles).

If you want to Assignment a Khan Academy video for your students to watch, then the easiest way to do it is to go to the video itself (as if you were going to watch it) and assign it from the blue bar at the top of the screen:


Just select which class you want to Assign it to (if you have more than one, choose individual students if you like, pick a date and time for the Assignment to expire, then click "Assign" (it's off-screen to the right in this example).

If you want to add a video, as in "record a video and have it added to Khan Academy" or "assign a non-K.A. YouTube video for students to watch", I'm afraid that's not possible.