Hello. I'll assume this is an older JVC, such an i-ART model, or the likes.
To hook-up a cable box or Roku, you will need a HDMI to Composite converter. Such as:
Amazon
This will allow you to view the HDMI signal on your tv set. Though this type of conversion isn't without issues. The text
WILL be small, and unfocused. It will also be in letter box format, so black bars on top and bottom. If it isn't in letter box, you will be missing text on the left and right.
Movies will look perfectly fine, but any TV Guide or Roku menu might be a hassle to browse.