Most Helpful Guy
They are copyrighted the moment you create them. What you want is a way to protect your copyright. If someone tries to steal your ideas, you need some way to show that you did it first, or at least did it independently. What you need will depend on how badly they want it, and to what lengths they will go to. If you want to be fully protected you'll need to talk to a lawyer. But even that might not be enough.
However for now, I wouldn't worry too much about it unless you are putting it online in public or something. Just having them in your possession will help. Your drawing style would be hard to forge also. But like I said, you have a right to it as soon as you create it. You really need to talk to a copyright lawyer if you get serious. If there is a law school near you (most cities have at least one) then there might be a program where you can talk to student lawyers for free. They are supervised and have real lawyers at their disposal to ask questions.
Most Helpful Girl
There's some controversy about copyrighting ideas. Some people claim once you come up with the idea, it's copyrighted. Copyrighting is a process in which you document your work and it's protected, in a way. As in, you own it, and if someone tries to steal it, you have proof. Smart people person go through the Library of Congress to register their work. That goes for photographs, written work, songs, etc.
You don't own an idea, though. However, you can own how it is executed. Meaning, if you convey that idea in a book or a poem or a comic strip, then you own the execution of that idea. IF you register it through the Library of Congress. Then there is proof that you executed the idea in a certain way, but if people want to use that same idea in a different way, they can.