Hi Tammy, okay...I've seen the responses with all kinds of needless questions so I will make this really easy!
1/. Just plug in the HDMI plug you got from cable box to HDMI port to TV... Use the HDMI 1 port on TV please for this step. After you plugged in both sides of the HDMI to its proper places then plug in box electrical wire to get power! Before turning on cable box turn on the TV and use the remote to get to the setting of the TV where u can see which channel line u are needing to use to get reception feed which should be HDMI 1.
Now once this is done turn on cable box... and wait till it goes through its setup! It would also be a good thing to make sure that the cable wire is in the right place for me its the back center of my box but for u it can be on the far left because there are two places on the xfinity box u can screw the cable two which could be the simple problem... as the TV and Box are both on try screwing on to either and give a few seconds for each to see if you get picture it should eventually work try the far left plug first though! And good luck!