If you pull up the Help, there should be a section titled something like "Getting started with vCenter collections." This covers the steps involved in planning your environment, configuring a machine to be a Managing Agent, configuring the settings, etc. In small and/or test environments you can use the Collector itself as a Managing Agent.
You'll need to verify the Collector is trusted and enabled as a Managing Agent (Certificates node). And, if you're not running HTTPS for the web console, you'll need to bypass the security that prevents entering sensitive information over an unsecure connection (Settings\General Settings\Collector) IIRC.
