It depends a little bit on the architecture of your application.
If your application is a client/server application that communicates via an HTTP POST type message, then you can use the LoadTesting plugin built into the Enterprise edition of TestComplete. Please read the appropriate help topics on that plugin in general as it will give you some good information as to how the feature is structured and the various objects available.
If your app does not communicate via HTTP POST, then you'll need to set up and operate a Network Suite with various tasks and jobs. This will require multiple licenses of either TestComplete or TestExecute in order to stage the multiple workstations. Please read the help topics on Distributed Testing and Network Suites for more information.
If you have specific questions once you get started, feel free to ask. There are plenty of folks up here doing a variety of similar tasks.