If it's an SDTV with only yellow/red/white connections you will need this "dongle":
If your SDTV is really old and only has a coax input you will need to convert the composite output from the dongle using a modulator like this one:
If it's an old HDTV with component inputs (red/green/blue) connections you will need this one:
You will of course also need the appropriate cables
You can get these items from several online stores, I am just using solid signal as an example.
You need to make very sure you get the correct dongle.
If DirecTv did this installation the installer should have provided the correct dongle.