Development:
Copyright and Your Ideas
Your Idea
Ideas cannot be copyrighted, and even if someone does "steal" your idea, their execution of it will be completely different from yours. Therefore, this is not something you should worry about.
Your Script
Copyright in the US is automatic from the moment you create a piece of work BUT to protect yourself and prove ownership you may wish to register the script with the US Copyright Office (or equivalent in another country) or with the WGA. Even non-WGA members may use this service. Registering with either the copyright office or the WGA costs money. Keep in mind that sharing scripts for feedback is one of the key ways that writers learn and improve so a certain element of trust is needed.
Disclaimer
This summarizes our understanding, however we are not lawyers so please contact an attorney who can advise you specifically on further questions. See Lawyers for the Creative Arts (LCA)