1. You can't protect your idea.
An idea itself is totally impossible to protect. There are 4 major types of intellectual property: copyright, trademark, patent, and trade secret. (There are outliers too: Canada protects circuit board designs for some weird reason for example. We'll focus on the big 4.) And in a nutshell:
- Copyright is the written expression of an idea and it protects the words or pictures used to express the idea. But if different words can express the same idea, copyright doesn't stop someone from doing that. Example: if you want to write a story about a group of underdogs who are fighting to overthrow an all-encompassing empire, that could be anything from Spartacus to Star Wars.
- Trademark is the specific words used in a phrase or the specific design used in a logo. Coke is The Real Thing, but if you want to make a soda and call it The True Thing then unless you try to trick people into thinking your soda is Coke then Coke probably can't stop you because it's not a copy.
- Patent is the method used to do a thing. If you can describe a set of steps used to do a thing, then you can patent those steps, but if someone can figure out another set of steps that produces the same thing then they're allowed to do it and you can't stop them.
- Trade secret is similar to patent, but it's... wait for it... a secret. The functional difference between the two: patents trap people who follow your steps even if they didn't know about your work but they expire after a period of time, and trade secrets don't trap other people but they also don't expire.
You may notice that I didn't describe anything that protects your idea. That's because it's pretty much impossible to protect a pure idea. The second you describe it, you create something that could inspire someone to make their own creation. And as far as the law is concerned, that's fine.
So if you want to make sure no one does something similar to your idea, unfortunately the only way to do that is not to share it. That's not practical. So instead, the best approach is to make sure that you only talk about it when it's ready to be shared. If you're writing a screenplay about an adventurer archaeologist, probably best to wait until you've got something pretty far along before you start shopping it around. It doesn't have to be a full draft. A treatment or some other summary that's sufficiently detailed to show whether someone has stolen the essence of your idea will be enough. (There are people who will tell you that you absolutely have to register your treatment at the Writers' Guild of America. I'll come back to why that's not a perfect solution in another post.)
2. The type of idea you have makes a huge difference in what you need to do to protect it.
Copyright is automatic: these blog posts are copyrighted by me the second I write them down, and there's no need for any registration or marking anywhere for that to happen. Trademarks can be registered but they don't need to be: continuous use in commerce and consistent indication of the mark with a (TM) is enough (that's the difference between (R) and (TM) if you were curious). Patents can only be protected if you file the patent application before you tell anyone about how to make your invention (except in some places where even if you file first, someone who figured it out first can defeat your application). And trade secrets... well, the name says it all.
So depending on your creation, sometimes you can go on at length without concern, sometimes you can talk about it in general, and other times you can't say anything at all. How can you know which one is your concern? Ask yourself: what would someone have to do for you to think they're stolen your idea to the point that it's not worth your working on it any longer. The description above of Star Wars: that story existed long before George Lucas came to it, and it can be retold with different characters in a different setting long after. But a version of it set in space, with a princess captured by a guy who wears a mask and invokes a power called the Force, which is also invoked by the good guys, and a space pirate who has a friend who's 7 feet tall and hairy... now you're getting to something that's sufficiently original that you could protect it.
One other thing. Lots of creative people work for companies in the creative industries. And lots of those companies have employment agreements that claim the employer owns all of your ideas. But that's not necessarily true. Why that is, and what you can do to benefit, in a future post.