Do you need to worry about someone stealing your idea?

Almost every new designer worries that sharing their game will get it stolen. The fear is understandable and almost always misplaced. Ideas are cheap in this industry. Every experienced designer has more ideas than they will ever finish, and turning an idea into a publishable game takes years of work that a thief would have to do themselves.

Legally, game mechanics cannot be copyrighted. Copyright protects your specific expression: your written rules, your art, your card text, and your game's name can be protected (names through trademark). The underlying systems, like worker placement or deck-building, are free for anyone to use, which is why the industry is built on games openly borrowing and improving each other's mechanisms.

The practical protection is community and paper trail. Playtest publicly, post development updates, and share your game widely. A hundred people who watched you build the game are better protection than a hundred NDAs. Publishers, for their part, have overwhelming incentive not to steal: their business depends on designers trusting them with submissions.

Be a little more careful only at the very end of the process, with a finished, proven design being pitched commercially. Even then, standard practice is simple records of what you sent to whom and when, not lawyers and secrecy.