I am selling it for free.
Heh.
Whether you're selling it or giving it away for free, what you're wanting is to protect your assets and application from being freely distributed, modified or sold by someone else.
I don't claim to be an expert about this - in fact I know nothing - but I think you need to display a EULA when downloading or installing the package, where the users clicks 'Yes I agree'. Additionally I would state copyright wherever you can, e.g. your software's title screen.
See Wikipedia's page on licencing:
http://en.wikipedia.org/wiki/Software_license