You can start an RP at any point. Just because your join date may be an hour ago doesn't mean that you aren't as good as someone who has been here for years. It just means that you're new to the site.

As long as you present your idea clear and well, people will become interested. Be aware though that in the case it doesn't work out it does not mean that it was a bad idea or that you don't have any cred. It could be the wrong timing (multiple of similar style), could be during school exam periods, or even it was bumped out of view by accident. In any case if you keep at it or try again in the future, it might work out.