The only thing you need to learn before MEAN stack is JavaScript. Because MEAN Stack is made up of technologies that all use JavaScript. Also you should be comfortable writing markup in HTML, style sheets in css and basic JavaScript and also a little bit of jQuery. If you wish to learn any of these things then you can do this HTML and jQuery training course. You can read this blog that explains what MEAN stack is.
Learn full stack web development in detail by signing up for this professional Full Stack Training offered by Intellipaat..
You can also get started with MEAN stack by learning individual technologies, like:
If you wish to learn MongoDB you can do this end to end certification course on MongoDB.
Also you can learn Angular by doing this Angular course.
or you can watch small tutorials to get your feet wet on MongoDB, Angular etc,