Hello @YaserHamame01.
What I can make out of this thread is that you are looking for a well structured course where someone holds your hand and leads you from a total noob to a full-stack/ front-end developer. I am afraid no such course exists though there are some which will pretty much expose you to the fundamentals of almost everything and then it is up to you to consolidate the knowledge gained. One of them is FCC. I suggest you start by identifying what it is that you want to learn, list them down and then start learning them one after another.
Regarding which one of the two is better between Udemy and Lynda, I donāt know. I suggest you look at each component of the MERN stack and look for courses specific for that component. For example if you want to learn react, search for react courses on Udemy or Lynda or even a free platform like FCC. Most of these paid learning platforms have ratings and reviews which can give you an idea of whether others who took the course before you found it useful or not. I believe it will give you an idea of which course will meet your expectations.
Finally the MERN stack is quite wide. My hones opinion is that it is difficult to find a course which covers all the stack in depth even if some claim to do so. I suggest you start with fundamentals of HTML, CSS and Vanilla JS . You can then pick up React and progress to Node/Express and MongoDB. If you do all of them at once I am afraid you will end up frustrating yourself.