Find if it is already been done. If you find it - contact the developer and figure out how you can help. If you don't find it, keep looking and start asking if

Find if it is already been done. If you find it - contact the developer and figure out how you can help. If you don't find it, keep looking and start asking if other people have heard of it. If you still don't find it, take a couple days off and then start looking again. Only after you've searched 3 times and couldn't find it - start your new project. The internet would be a better place if people were 50% as excited to contribute to an existing project as they are starting a new one.