is there a way to copyright a marketing idea
Sorry, but in the general case there is not. You should see this little article for an overview of the different types of intellectual property (IP) that exist, and which areas the different types of IP cover. But generally speaking, there is no legal exclusivity for abstract ideas.
how secure am I from idea theft once I do?
As note above, abstract ideas are not yours to own exclusively. So it would not be "theft" if someone else copies your idea.
That said, some concrete innovations can be patented, etc. The 'strength' of the protection varies by the type of IP as well as the specifics of each case. But in most cases, enforcing intellectual property claims means threatening to take or actually taking legal steps, and is both very expensive and time consuming.
where would I go to copyright something like that?
- First, see my linked article above for the specific types of IP.
- Read some of the past discussions under the relevant tags, to better your understanding of the topic.
- Go see a good, honest lawyer who specializes in intellectual property protection. This area is so complex that a layman will not be able to create a good IP defense strategy. And sadly, good IP lawyers are not cheap.