This mission line is a long one. A lot of the missions take quite some time to complete and require you to do some... interesting things. So be prepared to invest a fair amount of time here.
To start out, you must be playing as Michael. Only he can initiate these missions so switch to him now. Access his phone and visit the internet. Enter www.epsilonprogram.com and complete the Identity Evaluation. It doesn’t really matter how you answer because you will get a report that mentions a dented red truck either way. A “?” will appear on your map at Raton Canyon on a road between Mount Josiah and Cassidy Creek. This is the first mission in the line.