Adding Cable boxes to existing internet service? Help!

Currently, I have already ran my own coax from the street into the house all the way to my modem that is providing me internet. I'm looking to bundle TV and Internet and need to know what the process is adding the two TV's to my service. Is it as easy as adding a splitter in the attic turning my main trunk from the street into a three way?

TLDR: I have xfinity internet with my own cable ran, how do I add two TV's to my existing service?