The canon of invention has to do with the legwork; of coming up with good ideas. Invention entails doing research, (what is there to say on my topic, generally,) and narrowing the topic, or deciding one's "angle" (out of everything one could say, what do I want to say about this topic, at this time, with respect to this particular audience.
But why would this process be called "invention"? Because, when one is engaged in persuasive speaking or writing, one must invent, or construct, arguments. Invention is a very involved part of the art of rhetoric. It entails much more than merely deciding one's topic.
Okay. The first thing you need to do is gather information. I've made that a little easier for you here.
Here are some other helpful links:
Aristotle subdivides the art of inventing arguments into ethos, pathos & logos.
Two other important concepts: Stasis and Topoi