How would use the GPL? If i wanted to make a game open-source to the public? would it cost me any thing?

It doesn't cost anything. The instructions are here: http://www.gnu.org/licenses/gpl-howto.html

what about the copyright?

IIRC you still retain the copyright but that's relatively meaningless since under the GPL you allow other people to do whatever they want with your code as long as they also release it under the GPL. The only thing that being a copyright holder on a GPL released program gives you is the sole legal right to sue someone who uses your code but does not release their modified version under the GPL.

But if you really care go ask a lawyer.

-me

dont.

Make sure you understand the terms and ramifications of releasing your code under any license before doing so. Consult a lawyer or legal advisor (etc) if neccessary. Also note that the GPL isn't the only open-source license available to you -- there are others, such as the MIT or BSD license, the Boost license, etc. Google for "open source licenses" or something similar will yield many results.

no i mean i cant get a copyright, i dont know how to. Do i realy need one for something like this?

Elaborating on the "don'ts": your initial question implies that GPL == open source, which isn't the case. There's plenty of other licenses for opening the source of your software that you may wish to look at. The BSD, MIT and zlib licenses are quite popular too.

The GPL license is useful if you wish for anyone using your code to also release their code under the GPL - with all its pluses and minuses. Personally I don't like how the GPL is so politically charged and so the GPL is the kiss of death for any libraries I may want to use in my projects. This is a downside if your aim is to share your code with as many people as possible.

The BSD, MIT and zlib licenses (they're all quite similar) are somewhat more free of restrictions on how people use your code - all they're really forced to do is keep the license in the code that say it's yours. The plus side is that it's much easier for those who use your code, so they're more likely to use it. The downside is there's no obligation to release changes, so they might keep modified versions to themselves.

In the end it's your call, but make sure you know what your aims are for opening the source of your code and pick an appropriate license.

Edit: re copyright: You get copyright automatically once you create something in tangible form (i.e. it's not just in your head but in a form you can see, use etc.). Just by writing the program you automatically get copyright. Put a copyright reference like "Copyright <year> to <your name>" and you'll be fine.

In the US at least, copywritable works are copyrighted the moment you create them. By writing your code, you have produced something you (probably*) hold the copyright for.

You can register your copyright with the government for some nominal fee, but this is not necessary to actually have the copyright. A lawyer can advise you further as to whether this would be a useful action for you.

(*) The disclaimer would be that in some cases employment agreements or contracts signed with schools stipulate that you transfer the copyright to anything created with their tools or on their time to them, et cetera, et cetera.

When in doubt, consult a lawyer, not random strangers on the internet whose advice can very easily be wrong. Law is nontrivial.