I'm assuming you're from America.
Copyright is there to protect your ideas once they have a tangible form. Copyright immediately vests when you have a tangible representation of your idea. Ideas themselves are not copyrightable. You cannot copyright concepts (there is a long and interesting history of this in the case law).
The federal law controls copyright law, it is very very unlikely that your state law will apply (for reasons I won't explain because they're very very boring). Here is the Copyright Statute in its current form:
http://copyright.gov/title17/circ92.pdf This will be essentially useless to you. That's why copyright lawyers exist.
Selling a game, like selling anything, matters a lot on all sorts of issues we are not going to flesh out here. For example, will you be incorporating or forming a business entity? Or will you be working with an intermediary (think Steam). If you're just selling it yourself, then we need to know where? Again, won't get into this. Just remember that any proceeds will count as income for tax purposes.
if you're making a project, keep track of your resources very closely and make sure you're allowed to commercially use the ones you want to use. Once that is all sorted out, you can find an accountant or some professional who can point you in the right direction, or go through an intermediary like Steam, etc...
You wrote: "About trademarks, can they be used to reserve rights to a complete character?"
No. It's difficult, although not impossible, to have a copyright interest in a complete character (there is some case law I could show you if interested), trademark issues don't arise here; trademarks are for identifying products with a sign or symbol, or something like that.
This isn't legal advice ^ just some statements of fact
