Written guides
Even if youtube videos are now popular and can teach people with examples, those written guides are very detailed and usefull to start with.
Learn the META : Death Match by Furor_Teutonicus
DM Guide by [Quendi] Clemensor (PDF File)
Videos tutorials
Springs DM Intro : Short and Covers the very first questions
ImComings Introduction Videos : longer videos, more detailed
Spooly Gamings Channel : different levels of videos, also content for experienced DMe players on specific civs