I want to use an OTA HD antenna...

Lefty, yes you have the right idea. Your plasma most likely has a second coax input and HD ready. Unless it is a monitor and not a television per say.

You can buy a self powered antenna if you so choose. Or you can add a inline amplifier if necessary. 20 miles out isn't that far. Use the link I gave above to see how many channels are available in your area. You could in the future add additional antennas to the first one and have them to piggy back one another facing different directions in order to maximize your reception.

OTA digital is very directional..so your antenna must be pointed in the direction of where the signal originates to get the best signal strength.

You should be able to find a pole at Lowes or Home Depot. You could easily mount the pole in the attic using claps to your beams. Or in some cases you can get away with using an existing dish pole if the circumference of the pole is small enough.

Get an antenna that is made to be piggy backed. Radio shack carries one that does this for around $30. You'll need a pole of some sort. The antenna has claps for mounting to a small pole.

If you plan to use it with more than one or two TVs..I would get an inline amplifier/splitter or a self powered antenna/splitter.

I looked in my manual, and on the back of the TV. I have DISH connected through S-Video and composite plugs, and I do have an available Antenna-in co-ax connection. The manual has the external antenna connected to the cable box or the DVD, not directly to the TV. Do I need to do that, or can I run it directly into the Antenna-in on the TV? Do the roof mount antennas work a lot better than the table top ones I see for sale? For 20 miles, do I need an amplifier? Do I get all this at Radio Shack?

The table top models work well for some . It will not do as well as a roof or attic mounted antenna. Radio shack will have most of what you need. Walmart sells the table top versions fairly cheap. Try it without the amplifier first. You may not need it. You can get coax from Home depot reasonably.