Very simple. You write it, draw it or design it - the copyright is yours and you can sell it or licence its use. ( Unless you did so as part of your employment - different rules apply then.) You didn't create it - you can't use it in any way whatsoever without agreement and/or payment.
If you want further details, visit a Public Library and borrow a book on intellectual property rights. These vary from country to country, and there are International Conventions which may also apply.