Don’t worry about people stealing your ideas. If your ideas are any good, you’ll have to ram them down people’s throats.
Many designers are reluctant to share their work with others because they are afraid that someone will take their idea, make a fortune on it, and leave them out in the cold. These designers take one of two paths: Either they sit on their idea and try to develop it themselves, with no testing or help, and the idea never becomes marketable; or they spend time and money investing in documents like nondisclosure agreements (NDAs).
Here’s a fictive conversation with a student, some variation of which I have had dozens of times:
STUDENT: I have a great game ...