For an external antenna to pull in HD signals, your TV needs to have a digital (ATSC or QAM) tuner. If it doesn't have one already, you will need to buy an external digital tuner.
"HD Ready" meant different things in different TVs. For some TVs, it meant you could hook up an antenna, tell the TV to scan for channels, and you'd get whatever HD channels your local area had to offer. For others (like a JVC that I had), one had to buy a separate digital tuner box which connected to the TV (via either component or HDMI connection), hook up an external antenna, and then scan for channels.
If you hooked up the antenna and had the TV scan for channels and got nothing, it sounds like you're going to have to find an external digital tuner box. They used to be plentiful (mine was made by Panasonic), but with most TVs having built-in tuners now, I suspect these will be harder to find.
Good luck.