Since you don't mention which provider you have, we don't really have the actual starting point per se, since all of them have slightly different systems/hardware requirements.
That said, yes- it is likely you need to rent an additional box in order to receive the full quality. Not something you can usually buy from a third party to solve the problem, at least not now with the switch to digital & HDMI gear. Ask them.
Keep in mind that there are online forums where actual individuals from the respective providers can assist with this type of thing. Take a peek at the various sub-forums @ dslreports.com. FWIW, I have received fairly decent help there in the past when I was stuck.
I have a new Samsung 4k tv. I have a cable co. Dvr that will record 2 shows at a time that requires I go off box to watch another show. I have a Coax off a splitter to Coax input on the tv. Some channels are fine put many are not. The good channels are designated as an example 6-1 while the channel 6 is bad . Isn't this tv supposed to upscale any signal? Do I need some device to convert Coax to one of the other hdmi inputs for better quality on all channels?