This question goes a little deeper than can be explained here, as there are several different ways and/or reasons to side chain. I’m just going to cover the basics of how to make it happen; it’s up to you to decide what you want to do with it (gate trigger, ducking, de-ess, etc)
For this tutorial, you will need a few “givens”: a track to process and a track/source to be the trigger. Set the output of the trigger track to be an available bus. On the process track, insert the plugin that you are using that you want to side chain (compressor, gate, etc). Set the key input to be the bus you selected for the trigger track output, and activate the key. Make settings on the plugin accordingly.
An example of a use for a side chain gate trigger can be found in my other tutorial; How to fatten a kick drum in ProTools?
More information can be found starting on page 20 of the DigiRack Plug-ins Guide