Creating widgets for GNOME

So i have just installed Ubuntu GNOME on my system and would like to start coding some stuff for linux(all about that open source) so what im asking here is what language to use and has anyone got some tutorial projects to begin learning the system etc etc etc.

Thanks in advance

Well - I guess you mean extensions - do you? Those are in java.
Github has quite a lot of them - is a collection and you usually can find them on github under the same name