First of all, know what you can and cannot copyright:
You CANNOT copyright an idea in and of itself. Why? Consider this: many ideas, when broken down to their basic elements, revert to a handful of basic types. Stories of love and hate, conflict (versus self, versus other, versus environment, and whatever one or two others I'm forgetting), life and death.
Don't stop reading yet. What you CAN copyright is your specific expression of an idea. Let's just take an example here -- you want to write a story about two people who fall in love from 'different sides of the tracks,' so to speak. Yeah, it's been done before -- Romeo and Juliet is probably one of the most famous. Or consider the 'quest' story. Tolkien wasn't the first, by any means, to use this frame, but he was around late enough that, if he could copyright the entire quest idea, it'd be bad news for many, many authors and artists. I'm not even going to start listing -- you can do that on your own, just go over to the fantasy section of your library and bookstore and start taking names (okay, -slight- exaggeration).
Second...it is not necessary to pay to copyright your work (though there are certain benefits to that, I think?). Once it is out of your head and onto the paper/canvas/computer screen, it's yours. It's a good idea to include a basic copyright notice, however (example: Copyright 2005 {your name here}).
Last of all...I'm going to move this over to Comic Creators for you. They could probably answer your questions even more clearly over there.