I’ve come up with a brilliant idea. How do I stop someone from stealing it?

Well, it will depend what your brilliant idea is and who, what, where, when, why and how you aim to protect and exploit it. Perhaps start by looking at our legal guide on Intellectual Property Law (IP) and the sections on the four main IP law areas :  http://www.lawplainandsimple.com/legal-guides/article/understanding-intellectual-property-law