Write down all the things that excite and interest you. Think of your hobbies, your religion, your relationships and your philosophy on life. Use your personality and your interests to select things that other people may find informative.
Narrow down your topics by selecting ideas that inform your audience or deliver a message. According to the Toastmaster's website, it's important to choose a topic that you are knowledgeable about (inherently or through research) that other people have less knowledge about, which they will find interesting.
Choose a topic that is connected to anecdote or story. The easiest way to draw an audience into a topic is by opening with a short tale. If one of your brainstormed topics is connected to a joke, story or anecdote, it is likely to be a stronger speech topic.