There are all kinds of marketing (B2C, B2B, Demand Generation, Branding, online, offline, etc.) for all kinds of products/services and across an amazing array of markets, industries and consumers. Impossible, therefore, to provide a sensible answer to your question as it is posed. If you are referring to online marketing of a website, I would search on Google pretending to be a target customer for your products/services. Check out the sponsored ads (PPC) to see who is already selling and how they are promoting it on the sponsored ad. I would then Google the specific competitor's name to see where else they are promoting their business. This is not the "end all be all" approach, but it would be a good start for you, I suspect.