How do I start a language on Duolingo?
Is there a way I can start a language on Duolingo and if I can could I make up my own language and use that?
I'm not sure if you can create your own language and put it on here. But I think you can request to add languages if you contact someone like a moderator.
For making a course for your own language, try Memrise or TinyCards or Anki. These are all flashcard apps. I believe full sentences can be put into them all, so a course something similar in idea to Duolingo's courses can be built. If you haven't looked at how to create a language before, then zompist's Language Construction Kit is a good resource.
Duolingo really isn't set up for everyone to start making their own language.