It depends what you want to do. If you want to convert your continuous signal into a discrete one, so that you can feed it to a (discrete) controller for example, then use a Rate Transition block. If you just want to log the signal to the workspace at a sampling frequency of 1 kHz, then simply use a To Workspace block with the relevant sample time.
More details on logging and other options are available in the Simulink documentation under Visualizing and Comparing Simulation Results and Importing and Exporting Data.
