First stages in Development: Language selection

I have been learning on treehouse for only a short time, and while I know that there is much more I need to learn, I need to decide upon the right path to take in regards to language selection. At the moment I am learning JavaScript, and although the concepts make sense I am having trouble seeing how I am to implement this into making an actual product in mind.

I can understand how I would write a page made up of HTML, CSS and JavaScript, then add these files to a server to see them functioning live on the web, but how would I go about creating a site/app that is for the following:

I would be creating a tool that allows anyone to use or download, but it will require a paid user to create a private network to which users have to be approved into. Then the admin("the paid user") can post information out to various subgroups within their network and the receiver get a push notification about the update.

The interface would be a bulletin containing all information relevant to that group. The admin interface will be a timeline (or some account) of all the things they have posted out to people to help them stay clear about what they have posted, pending or completed. The admin can choose to add/remove information as necessary, create priority, add remove users etc. It would need to work on all device types so I guess would look at going cross platform in the early stages and later aiming to provide native applications as per device.

Just like many people on here I guess I have many ideas I would like to make a reality and while treehouse is helping me to understand code, its raising new questions about the actual implementation, and even knowing what languages to master in the first place. I guess its a really common question to ask "what languages do I need to use to build my project" but I am concerned that I will be spending much time learning one language (say JavaScript) while I really need to be focusing down on PHP, or even Ruby for the paid user interface for example. I feel its such a vast subject one can spend much time going in a direction which could make it more awkward in achieving the end goal.

I guess I am just looking for a general idea of the kind of direction I need to be first headed in.

Most of my ideas revolve around productivity where I will need to take advantage of a network type structure sending, receiving and updating document information, allowing transactions from users, and push notifications. Then in some other unrelated cases creating event schedules, geo locations, hosting league table and leader boards.

My understanding is that I will need a database (SQL) to hold information at which some users can access and others can edit and that is about it. Can someone help to paint this logic into a full picture that I can understand? Maybe then I can decide on the one or two languages I really going to need and master those.

Any answer to the following would be much appreciated:

What language or combination of languages/frameworks is the right scope for my requirements? How does one determine the right languages to build their tools in? Is there a place to go where such questions can be answered, or the fundamentals of this process are clearly explained? Or do I even need to know all the code, or can I write parts and use others snippets? Is Github one of these places where I can pull fragments of code useful for these functions and combine them to get the desired results?