MERN Stack consists of four technologies.

M - MongoDB
E - Express
R - React
N - Node

All these technologies are built on JavaScript. To learn JavaScript these are the best resources.

https://www.Javascript.info

Jonas schmedtmann Udemy Course
https://www.udemy.com/course/the-complete-javascript-course/

After you have learnt JS then proceed to learn Node.js

Node.js is a backend language used for highly scalable applications. Many companies have shifted to Node.js for the scalability and performance provided by Node.js

After learning Node, you should learn Express. Express is a framework built on Node.js. It is very lightweight and easy to learn. Simplifies development with Node.js.

After Learning Express the next step is to learn MongoDB and Mongoose. MongoDB is a NOSQL database and Mongoose is a library that can be used with MongoDB.

You can learn Node, Express and MongoDB from these Udemy courses. They are the best till date.

1) Jonas Schmedtmann Udemy Course
https://www.udemy.com/course/nodejs-express-mongodb-bootcamp/

2) Mosh Hamedani Udemy Course
https://www.udemy.com/course/nodejs-master-class/

So, after learning Node, Express and MongoDB you’re already a Backend Developer. All that's left is to learn is the Front End.

Now you should proceed to learn React. React is a frontend library that is developed and maintained by Facebook. It is super lightweight and is very powerful in performance for any web app. To learn React there are many resources available for free on the web. Also, after learning React you should learn Redux. Redux is state management tool used with React Apps. My personal favourite resources for learning React and Redux are the following.

CodeVolution YouTube Playlist
https://www.youtube.com/watch?v=QFaFIcGhPoM&list=PLC3y8-rFHvwgg3vaYJgHGnModB54rxOk3

Official React Docs
https://reactjs.org/docs/getting-started.html

Stephen Grider Udemy Course
https://www.udemy.com/course/react-redux/

Learnt All of these technlogies ? Done. You're A MERN Stack Developer.

Next Step :- Develop some projects to showcase in you portfolio.

Give a like to the post if this helps you out. Also comment down the resources from which you have learned MERN Stack to help others out.

The author of the article is the founder of startup Shashaan Web Solutions. Do check it out. https://www.shashaan.com