Why would you allow your decision to be driven by the limitations of a hosting account? MySQL is a relational database, and typically you need a solid understanding of relational database design and SQL. MongoDB as a so called "Document database" is not relational. You can associate several MongoDB collections together if you understand the basic concepts behind, it, but in general, MongoDB does not have nor require a fixed schema. It can do many of the things a relational database can do, particularly if your data fits nicely into a hierarchical structure. It also often appeals to people like yourself that are working with a javascript stack, as the interface to MongoDB is json. If you can install a package onto your host, then you should be able to use MongoDB. With that said, you can do all your development locally using Docker containers. I would not suggest that you let important design decisions be dictated by what one shared host allows you to do.
Start with C. C is a language that is still widely used for operating system and library development, as well as embedded systems. It's also a language that requires direct management of memory. It is without a doubt the single most influential computer language, in that much of the syntax you learn has been borrowed widely by , which will make learning other languages much easier in the future. It's also the base for C++, which is an extension of C. Learning C will teach you many computer science concepts, that will allow you to compare and contrast it with the design and features of other languages. As far as getting a job out of high school, that very much depends on the type of development you want to pursue. For example, if you'd like to get a job in the world of web development, perhaps as a "Frontend developer" none of the languages you listed will allow that, without a lot of other conceptual knowledge and a good working understanding of html and css, to go along with your javascript (and most likely, an understanding of a javascript framework like react or vue).