I have been observing and looking at MongoDB for a couple of years already but never used it in any really serious or professional context. I am a content PostgreSQL user since 2005 and exposed to Oracle for more than 15 years. MongoDb as a NoSQL database showed up in 2009, it is a free and open-source cross-platform document-oriented database, it breaks with the traditional relational database concept. You can read more about its history and the key facts at Wikipedia.
Here is a good overview of the key facts of the current NoSQL landscape products.
Last year release 3.0 came out in 2015 with a significant number of improvements and changes. Time to spend time on it again and jump straight into the 3.x release world. I noticed a lot of books, tutorials and websites refer to the earlier versions and unfortunately using deprecated functions and features.
It takes very little to get started. Download and install it on Windows, Linux or Mac and get you hands dirty with the DB shell, which I recommend to do for the beginning to pick up the basics and concepts, before using a GUI. Free yourself from the relational DB way of thinking, try to adopt the concept of documents instead. There is also a number of books about MongoDB out there, but the website is quite rich in info and you find myriads of people talking about it in blogs and forums.
When you look for a GUI, there a couple of them, some free, some commercial, I recommend to have try with 3T’s MongoChef. Though it is commercial (decent price tag) it is free to use for non-commercial (!) activities.
Here is a good page with an overview of current tools.