1. MongoDB University
The first and the best site to learn MongoDB out there. Why? Because it is being taught by the same company who created MongoDB in the first place! Awesome isn't it?
You can check out there catalog of online courses here: https://university.mongodb.com/courses/catalog
If you are a complete beginner and have just dived into databases and all, you'd go with their "M001: MongoDB Basics" course. It will help you get a head start with MongoDB - its working and basis as the name suggests.
If you are an existing developer and already using other technologies like Java, .NET or Node.js, you can opt for their other courses which are designed specifically for such developers.
2. StudyTonight - MongoDB
Instead of having a course in form of videos, this portal offers tutorials with informational pictures. The MongoDB is divided into three sections: Introductions - Basics - Advanced
Hence you can start with the intro and then move up through topics as you feel comfortable.
You can get an overview of the different topics in the tutorials by this picture.
To access the website go here: www.studytonight.com/mongodb/
3. TutorialsPoint - MongoDB
TutorialsPoint is one of the most exhaustive resources for ANY language, platform or framework out there and it is still growing. The navigation is easy, user interface is good and all the topics are named for easy access and usage.
Same goes for the MongoDB tutorials too. From setting up the environment correctly to creating & analyzing complex queries, you will find yourself browsing and learning new concepts in no time.
You can access the website here: https://www.tutorialspoint.com/mongodb/index.htm
I hope this post helped you a bit!